UC-NRLF 


AN   INTRODUCTION 

TO 

GEOLOGY 


\ 


INNER  GORGE  OF  THE  GRAND  CANON  OF  THE  COLORADO.     (U.  S.  G.  S.) 

Frontispiece 


AN     INTRODUCTION 


TO 


GEOLOGY 


BY 


WILLIAM    B.    SCOTT 

BI.AIR   PROFESSOR  OF.GEOLOGY  AND   PALEONTOLOGY 
IN    PRINCETON    UNIVERSITY 


'  There  roils  the  dee p  where  grew  the  tree. 
O  earth  what  changes  hast  thou  seen ! 
There  where  the  long  street  roars,  hath  been 
The  stillness  of  the  central  sea. 

1  The  hills  are  shadows,  and  they  flow 

From  form  to  form  and  nothing  stands; 
They  melt  like  mists,  the  solid  lands, 
Like  clouds  they  shape  themselves  and  go." 


STeto  gorfe 
THE    MACMILLAN    COMPANY 

LONDON:  MACMILLAN  &  CO.,  LTD. 
IQOO 

All  rights  reserved 


Q 


f 


COPYRIGHT,  1897, 
BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped  March,  1897.      Reprinted  January, 
1898;  August,  1899;  November,  1900. 


NovfajoolJ  ^krss 

J.  S.  Cashing  &  Co.  —  Berwick  &  Smith 
Norwood  Mass.  U.S.A. 


It 

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TO 

A.   A.   P.   S. 
Book  i&  BrtucatetJ 

IN   GRATEFUL   RECOGNITION    OF   AN    EVER    READY 
AND   INSPIRING   SYMPATHY 


221733 


PREFACE 

THIS  book  had  its  origin  in  the  attempt  to  write  an  introductory 
work,  dealing  principally  with  American  Geology,  upon  the  lines 
of  Sir  Archibald  Geikie's  excellent  little  "  Class- Book."  In  spite 
of  vigorous  efforts  at  compression,  it  has  expanded  to  its  present 
size,  though  the  difference  from  the  "  Class-Book,"  in  this  respect, 
lies  not  so  much  in  the  quantity  of  matter  as  in  the  larger  size  of 
the  type  and  illustrations. 

The  book  is  intended  to  serve  as  an  introduction  to  the  science 
of  Geology,  both  for  students  who  desire  to  pursue  the  subject 
exhaustively,  and  also  for  the  much  larger  class  of  those  who  wish 
merely  to  obtain  an  outline  of  the  methods  and  principal  results 
of  the  science.  To  the  future  specialist  it  will  be  of  advantage  to 
go  over  the  whole  ground  in  an  elementary  course,  so  that  he 
may  appreciate  the  relative  significance  of  the  various  parts,  and 
their  bearing  upon  one  another.  This  accomplished,  he  may 
pursue  his  chosen  branch  much  more  intelligently  than  if  he  were 
to  confine  his  attention  exclusively  to  that  branch  from  the  begin- 
ning of  his  studies. 

Students,  and  only  too  often  their  instructors,  are  apt  to  prefer 
a  text-book  upon  which  they  can  lean  with  implicit  confidence, 
and  which  never  leaves  them  in  doubt  upon  any  subject,  but  is 
always  ready  to  pronounce  a  definite  and  final  opinion.  They 
dislike  being  called  upon  to  weigh  evidence  and  balance  proba- 
bilities, and  to  suspend  judgment  when  the  testimony  is  insufficient 
to  justify  a  decision.  This  is  a  habit  of  mind  which  should  be 
discouraged ;  for  it  deludes  the  learner  into  the  belief  that  he 
knows  the  subject  when  he  has  only  acquired  some  one's  opinions 


X  PREFACE 

and  dogmas,  and  renders  further  progress  exceedingly  difficult  to 
him.  In  no  science  are  there  more  open  questions  than  in  Geol- 
ogy, in  none  are  changes  of  view  more  frequent,  and  in  none, 
consequently,  is  it  more  important  to  emphasize  the  distinction 
between  fact  and  inference,  between  observation  and  hypothesis. 
An  open-minded  hospitality  for  new  facts  is  essential  to  intel- 
lectual advance. 

The  order  in  which  the  different  sections  of  the  book  are  taken 
up  should  depend  somewhat  upon  the  season  of  the  year  in  which 
the  study  is  begun.  The  chapter  on  the  Rock-forming  Minerals  is 
intended  rather  for  reference  than  for  actual  learning,  and  should 
at  first  be  employed  only  to  give  the  beginner  a  notion  of  what 
minerals  are  like  and  to  familiarize  him  with  a  few  of  the  com- 
monest and  most  important  kinds.  The  unfortunate  likeness  in 
the  terminations  of  the  names  of  so  many  minerals  and  rocks  is  a 
source  of  great  confusion  to  the  beginner.  It  is,  therefore,  impor- 
tant that  he  should  have  grasped  the  conception  of  what  a  mineral 
is,  before  commencing  to  deal  with  rocks.  A  repeated  experience 
of  this  confusion  has  led  to  the  wide  separation  of  the  chapter 
upon  minerals  from  those  treating  of  the  rocks.  It  is  perhaps 
hardly  necessary  to  say  that  a  knowledge  of  elementary  inorganic 
Chemistry  is  indispensable  to  an  understanding  of  almost  any  part 
of  Geology,  and  especially  of  those  parts  which  are  concerned  with 
the  minerals  and  rocks. 

If  the  course  of  study  be  commenced  in  the  autumn,  it  will  be 
well  to  take  up  first  the  chapters  upon  the  Surface  Agencies,  or 
even  the  Structural  Part,  according  to  the  opportunity  for  out- 
door work  and  occasional  excursions.  When  it  is  possible  to 
undertake  it,  this  work  in  the  field  should  by  no  means  be 
omitted.  Even  for  those  who  have  no  intention  of  becoming 
geologists,  observation  at  first  hand  possesses  a  far  higher  interest 
and  charm  and  a  much  greater  educational  value  than  merely 
reading  books  or  hearing  lectures.  Such  observation  is  also  a 
corrective  of  the  false  impressions  which  are  necessarily  given  by 
the  somewhat  artificial  and  systematic  treatment  of  a  vast  subject 
in  a  text-book.  In  many  cases,  it  is  impracticable  for  the  teacher 


PREFACE  XI 

to  take  his  class  into  the  field.  Under  these  circumstances  he 
should  constantly  impress  upon  the  minds  of  his  pupils  the  inade- 
quacy of  all  schemes  and  systems  to  embrace  the  great  facts  of 
nature,  and  should  encourage  them  to  observe  for  themselves, 
testing  what  they  read  by  what  they  see. 

In  preparing  this  book,  I  have  of  course  availed  myself  of 
material  wherever  it  was  to  be  found,  but  I  wish  to  acknowledge 
my  special  obligations  to  the  text-books  of  Dana,  Le  Conte, 
Geikie,  Green,  Prestwich,  Credner,  Kayser,  Neumayr,  Koken,  de 
I/ippirent,  and  Jukes-Brown.  From  the  last-named  writer  is 
taken  the  arrangement  of  the  Dynamical  Agencies,  which  expe- 
rience in  the  class-room  has  led  me  to  consider  as  the  best. 
Besides  these  general  works,  I  have  received  great  help  from 
monographs  and  special  articles  by  many  writers,  particularly 
from  those  by  Clark,  Cross,  Dale,  Dean,  W.  M.  Davis,  Gilbert, 
Harris,  Kemp,  Russell,  Van  Hise,  Walcott,  Willis,  Weed,  and 
others. 

I  take  sincere  pleasure  in  acknowledging  the  extremely  kind 
and  ready  assistance  which  many  fellow- workers  in  all  parts  of  the 
country  have  granted  me  with  unsparing  liberality.  Mr.  Walcott, 
Director  of  the  United  States  Geological  Survey,  has  been  espe- 
cially kind  in  this  respect,  and  has  allowed  the  fullest  use  of  the 
Survey's  fine  collection  of  photographs.  The  liberal  way  in  which 
advantage  has  been  taken  of  this  permission  is  to  be  seen  in  the 
many  illustrations  in  the  following  pages  marked  (U.  S.  G.  S.),  all 
of  which  were  made  from  the  Survey  photographs.  Many  other 
members  of  the  United  States  Geological  Survey  have  spared  no 
pains  to  help  me  in  the  work  of  compilation,  with  advice,  infor- 
mation, papers,  drawings,  photographs,  and  every  other  means  in 
their  power.  To  these  gentlemen  my  obligations  are  very  great, 
and  to  Messrs.  Walcott,  Cross,  Emmons,  Gilbert,  Hill,  Weed,  and 
Willis  I  wish  to  express  my  cordial  thanks  for  many  acts  of  kindly 
and  most  valuable  assistance. 

Professor  J.  F.  Kemp  was  so  kind  as  to  send  me  the  advance 
sheets  of  his  "  Lecture  Notes  on  Rocks,"  of  which  extensive  use 
has  been  made.  Mr.  A.  Smith  W'oodward,  Mr.  Agassiz,  Dr.  Bash- 


Xll  PREFACE 

ford  Dean,  and  Professor  I.  C.  Russell  have  kindly  supplied  me 
with  illustrations  from  their  books.  Mr.  Lucas  of  the  U.  S. 
National  Museum,  Dr.  C.  Hart  Merriam  of  the  U.  S.  Agricultural 
Department,  Professor  R.  D.  Salisbury,  Professor  Calvin  of  the 
Iowa  Geological  Survey,  Mr.  Pynchon  of  Hartford,  and  the  offi- 
cers of  the  Pennsylvania  Railroad  Company  have  furnished  many 
valuable  photographs.  My  colleagues,  Professors  Magie  and  Lib- 
bey,  have  assisted  me  with  the  proofs,  and  the  latter  has  allowed 
the  free  use  of  his  collection  of  unpublished  photographs  taken  in 
Greenland,  Alaska,  and  the  Hawaiian  Islands.  Another  colleague, 
Dr.  A.  E,  Ortmann,  has  taken  great  pains  in  the  selection  of 
figures  of  the  American  fossil  invertebrates,  which  have  been  re- 
drawn by  Mr.  R.  Weber,  University  Draughtsman.  My  friend, 
Dr.  Baur,  has  been  my  guide  through  the  tangled  mazes  of  the 
synonymy  of  the  American  fossil  reptiles.  To  these  gentlemen, 
one  and  all,  hearty  gratitude  is  due  for  oft-repeated  and  unstinted 
kindness. 

No  one  can  be  more  conscious  than  the  author  of  the  very 
imperfect  character  of  his  performance,  but  he  ventures  to  hope, 
nevertheless,  that  the  book  may  find  a  place  of  usefulness,  supple- 
mentary to  the  host  of  excellent  works  on  Geology  already  in 
existence. 

PRINCETON,  N.  J.,  Jan.  15,  1897. 


CONTENTS 


PAGE 

INTRODUCTION        ....         .......         i 

CHAPTER   I 

THE  ROCK-FORMING  MINERALS     .  ......        8 

PART  I 

D  YNAMICAL    GE  OLOGY 


SUBTERRANEAN  OR   IGNEOUS   AGENCIES 

CHAFFER    II 
INTERIOR  CONSTITUTION  OF  THE  EARTH  —  VOLCANOES         ...      31 

CHAPTER   III 
EARTHQUAKES  —  CHANGES  OF  LEVEI  ........      61 

SECTION  II 
SURFACE  AGENCIES 

CHAPTER    IV 
DESTRUCTIVE  PROCESSES  —  THE  ATMOSPHERE       .....      72 

CHAPTER   V 
DESTRUCTIVE  PROCESSES  —  RUNNING  WATER        .....      83 

CHAPTER  VI 

DESTRUCTIVE  PROCESSES  —  ICE,  THE  SEA,  LAKES         ....     104 


xiv  CONTENTS 

CHAPTER   VII 

PAGE 

RECONSTRUCTIVE  PROCESSES  —  LAND,  SWAMP,  AND  RIVER  DEPOSITS  .     124 

CHAPTER   VIII 
RECONSTRUCTIVE  PROCESSES  —  LAKE  AND  ICE  DEPOSITS      .        .        .143 

CHAPTER  IX 

RECONSTRUCTIVE  PROCESSES  —  MARINE  AND  ESTUARINE  DEPOSITS       „     160 

PART  II 

STRUCTURAL    GEOLOGY 

CHAPTER   X 
THE  ROCKS  OF  THE  EARTH'S  CRUST  —  IGNEOUS  ROCKS       .        .        .     186 

CHAPTER  XI 
THE  SEDIMENTARY  ROCKS 204 

CHAPTER   XII 

THE  STRUCTURE  OF  ROCK  MASSES  —  STRATIFIED  ROCKS     .        .        .     218 

CHAPTER   XIII 
DISLOCATIONS  AND  FRACTURES  OF  STRATA   ......     243 

CHAPTER   XIV 
CLEAVAGE,  JOINTS,  MINERAL  VEINS,  UNCONFORMITY    ....     260 

CHAPTER  XV 

UNSTRATIFIED  OR  MASSIVE  ROCKS        ....  .  274 

CHAPTER   XVI 
METAMORPHISM  AND  METAMORPHIC  ROCKS  ......    287 


CONTENTS  XV 

PART  III 

PH  YSW  GRA  PIIICA  L    GEOLOGY 
CHAPTER    XVII 

PAGE 

LAND  SCULPTURE  ...  300 

CHAPTER   XVIII 
ADJUSTMENT  OF  RIVERS 321 

CHAPTER   XIX 
MOUNTAIN  RANGES  —  CYCLES  OK  EROSION 332 

PART  IV 
HISTORICAL    GEOLOGY 

CHAPTER   XX 
FOSSILS 343 

CHAPTER   XXI 

ORIGINAL  CONDITION  OF  THE  EARTH —  PRE-CAMRRIAN  PERIODS        .    356 

CHAPTER   XXII 
THE  PALAEOZOIC  PERIODS  —  CAMBRIAN 365 

CHAPTER  XXIII 
THE  ORDOVICIAN  (OR  LOWER  SILURIAN)  PERIOD        ....     375 

CHAPTER   XXIV 

THE  SILURIAN  (UPPER  SILURIAN)  PERIOD    .  .  385 

CHAFFER   XXV 
THE  DEVONIAN  PERIOD 394 


xvi  CONTENTS 

CHAPTER   XXVI 

PAGE 

THE  CARBONIFEROUS  PERIOD 408 

CHAPTER   XXVII 
THE  PERMIAN  PERIOD    . .     428 

CHAPTER   XXVIII 
THE  MESOZOIC  PERIODS  —  TRIASSIC      .......    441 

CHAPTER   XXIX 
THE  JURASSIC  PERIOD   .        .        .        . 457 

CHAPTER   XXX 
THE  CRETACEOUS  PERIOD 474 

CHAPTER   XXXI 
CENOZOIC  ERA  —  TERTIARY  PERIOD      .......    494 

CHAPTER   XXXII 

THE  QUATERNARY  PERIOD  (OR  PLEISTOCENE) 525 

APPENDIX   I 
TABLES  OF  EUROPEAN  GEOLOGICAL  FORMATIONS  .        .        .        .        .541 

APPENDIX   II 

CLASSIFICATION  OF  ANIMALS  AND  PLANTS    ......     545 


INDEX  .............     551 


LIST   OF   ILLUSTRATIONS 


FIG.  PAGE 

Inner  Gorge  of  the  Grand  Canon  of  the  Colorado     .         .        Frontispiece 

1.  Forms    of     the    Isometric    System:    Cube,    Regular    Octahedron, 

Rhombic  Dodecahedron 9 

2.  Forms  of  the  Tetragonal  System  :  Right  Square  Prism,  Square  Octa- 

hedron   9 

3.  Forms  of  the  Hexagonal  System:    Hexagonal  Prism,  Khombohe- 

dron,  Scalenohedron     .                  10 

4.  Forms  of  the  Orthorhombic  System:   Rhombic  Octahedrons     .         .  10 

5.  Forms  of  the  Monoclinic  System:   Monoclinic  Pyramid  and  Prism    .  10 

6.  Profiles  of  Krakatoa,  before  and  after  the  eruption  of  1883       .         .  38 

7.  Crater  Lake,  Oregon 39 

8.  Crater-floor  of  Kilauea,  showing  the  lava  lake,  Hale-mau-mau          .  40 

9.  Another  view  of  the  crater-floor  and  walls  of  Kilauea       ...  41 

10.  Edge  of  Hale-mau-mau,  showing  the  ropy  forms  of  lava  ...  42 

11.  Lava  flow  of  Vesuvius       . 43 

12.  Lava-tunnel,  and  "  spatter-cone  "  formed  by  escaping  steam,  Kilauea  44 

13.  Lava  stalactites  and  stalagmites,  in  lava-tunnel,  Kilauea  ...  45 

14.  Stream  gorge,  island  of  Hawaii;   displaying  modern  columnar  lava  .  48 

15.  Obsidian  Cliff,  Yellowstone  Park.     Hexagonal  jointing     ...  49 

16.  Pompeii,  showing  depth  of  volcanic  accumulations  ....  52 

17.  Mauna  Loa,  seen  from  a  distance  of  40  miles            ....  53 

18.  Mt.  Shasta,  California 54 

19.  Vesuvius  and  Monte  Somma    ........  55 

20.  Truncated  tuff  cone,  island  of  Oahu          ......  56 

21.  Falls  of  the  Snake  River 57 

22.  Excavation,  displaying  the  transition  from  rock  below  to  soil  above  77 

23.  Bad  lands  of  South  Dakota 79 

24.  Bad-land  peak,  South  Dakota 80 

25.  Cliffs  and  talus  slope,  Delaware  Water  Gap,  Pa 81 

26.  Shales  "  creeping  "  under  the  action  of  frost 82 

27.  Weathered  and  exfoliating  granite,  Sierra  Nevada,  California   .         .  84 

28.  Diagram  illustrating  how  surface  and  underground  drainage  may  be 

in  opposite  directions  .         . 89 


XV111  LIST  OF  ILLUSTRATIONS 

FIG-  PAGE 

29.  Natural  Bridge,  Virginia 91 

30.  Arrangement  of  strata  which  causes  hillside  springs          ...       92 

31.  Diagram  of  fissure-spring 93 

32.  Au  Sable  Chasm,  N.Y 99 

33.  The  Dalton  Glacier,  Alaska      ........     105 

34.  Crevasse  in  a  glacier,  partly  concealed  by  a  snow-bridge  .         .         .     107 

35.  Vegetation  growing  on  the  Malaspina  Glacier 108 

36.  Nunatak  rising  through  the  ice  cap,  Greenland         ....     109 

37.  Edge  of  the  Greenland  ice-sheet,  with  a  glacier  descending  from  it .     no 

38.  Rock  polished  by  glacial  ice,  near  Englewood,  N.J.          .         .         .in 

39.  Scored  and  smoothed  limestone  from  Montreal,  Canada  .         .         .     in 

40.  Side  of  trough  cut  by  glaciers  in  the  limestone  of  Kelly's  Island       .     112 

41.  Front  of  Bowdoin  Glacier,  Greenland       .         .         .         .         .         .114 

42.  A  rocky  shore,  coast  of  Maine 117 

43.  Coast  of  Scotland,  showing  effects  of  marine  erosion        .         .         .118 

44.  Old  lake  terraces,  western  New  York 120 

45.  Beach  on  Lake  Ontario   .  121 

46.  Sand  dune  on  the  coast  of  Rhode  Island 126 

47.  Sand  dune,  showing  wind-ripples 126 

48.  Ideal  section  through  Mammoth  Hot  Springs  .         .         .         .         .127 

49.  Travertine  terrace  of  the  Mammoth  Hot  Springs,  Yellowstone  Park     128 

50.  Crater  of  Castle  Geyser,  Yellowstone  Park 129 

51.  Great  Dismal  Swamp 134 

52.  Alluvial  cone,  Wasatch  Mountains,  Utah 138 

53.  Mountains  nearly  buried  under   old   lake  deposits;    plain  of  Salt 

Lake,  Utah 144 

54.  Map  of  Lake  Bonneville 147 

55.  Island  of  calcareous  tufa,  Pyramid  Lake,  Nevada     .         .         .         -150 

56.  Calcareous  deposits  in  Mono  Lake,  California 151 

57.  Glacier  des  Bossons,  Switzerland 153 

58.  Perched  block  of  sandstone  resting  on  trap,  Palisade  Ridge,  N.J.    .     154 

59.  Perched  block,  near  the  Yellowstone  Canon,  National  Park      .         .     155 

60.  River  issuing  from  the  Malaspina  Glacier,  Alaska     .         .         .         .156 

61.  The  Chaix  Hills,  Alaska.     Moraine  material  stratified  by  water        .     157 

62.  Deposit  partly  made  by  stranded  ice,  west  coast  of  Greenland  .     158 

63.  Basin  of  the  Gulf  of  Mexico .162 

64.  Littoral  deposits  on  the  west  coast  of  Greenland      .         .         .         .163 

65.  Diagram  illustrating  the  change  of  materials  on  the  sea-bottom         .     164 

66.  Patch  of  corals  on  the  Great  Barrier  Reef  of  Australia     .         .         .167 

67.  Corals  on  the  Great  Barrier  Reef  of  Australia  .         .         .         .     168 

68.  Various  forms  of  modern  coral  limestone         ••.         .         .         .         .169 


LIST  OF  ILLUSTRATIONS  xix 


69.  Modern  shell  limestone  (Coquina)  from  Florida     .         .         .         .  171 

70.  Ancient  limestone  composed  of  various  kinds  of  organisms    .         .172 

71.  Rock  from  Pourtales  plateau 173 

72.  Map  of  marine  deposits  in  the  western  Atlantic      .         .         .         .177 

73.  Foraminiferal  ooze.      X  20 178 

74.  Pteropod  ooze.      X  4 179 

75.  Diatom  ooze.      X  150 180 

76.  Chalk  from  Kansas,      x  45 213 

77.  Section  in  coal  measures  of  western  Pennsylvania  ....  220 

78.  Sections  near  Colorado  Springs       .......  222 

79.  Cross-bedded  sandstone 223 

80.  Ripple  marks  on  a  modern  sea  beach 224 

81.  Wave  marks  and  rain  prints,  modern  sandy  beach  .         .         .         .  225 

82.  Rill  marks  on  modern  sandy  beach 226 

83.  Sun  cracks  in  sandstone 227 

84.  Markings  by  marine  worms,  modern 228 

85.  Tracks  of  land  animal  and  sun-cracks  on  slab  of  sandstone   .         .  228 

86.  Large  concretions,  weathered  out  of  sandstone,  near  Fort  Buford, 

Montana 229 

87.  Ironstone  concretion,  Mazon  Creek,  Illinois 230 

88.  Clinometer 232 

89.  Diagram  explanatory  of  dip  measurement 233 

90.  Anticline  on  the  Potomac,  Maryland 234 

91.  Anticlinal  limb  of  fold 235 

92.  Synclinal  limb  of  fold 235 

93.  Anticlinorium;  section  through  the  Appalachian  Mountains  .         .  236 

94.  Synclinorium,  Mt.  Greylock,  Massachusetts 236 

95.  Diagrams  of  folds :   I.  Upright  or  symmetrical  open  folds.   2.  Asym- 

metrical fold,  open.     3.  Asymmetrical  fold,  closed  and  over- 
turned.     4.   Symmetrical    fold,    closed.      5.  Closed    anticline, 

overturned.     6.  Closed  anticline,  recumbent       ....  237 

96.  Asymmetrical  open  fold,  High  Falls,  Ulster  County,  N.Y.       .         .  238 

97.  Symmetrical  closed  anticline,  near  Quebec,  Canada         .         .         .  239 

98.  Closed,  recumbent  folds,  Doe  River,  Tennessee       ....  240 

99.  Inclined  isoclinal  folds,  eroded 241 

100.  Diagram  of  monoclinal  fold 241 

101.  Section  through  faulted  beds 244 

102.  Normal  fault  of  small  throw  in  horizontal  strata       ....  245 

103.  Strata  bent  upward  near  the  fault  plane  .         .         .         .         .         .  247 

104.  Abert  Lake,  Oregon.     The  line  of  cliffs  is  a  fault  scarp  .         .         .  248 

105.  Effect  of  strike  fault  on  outcrop       .        •        •        .        .        .        .  249 


XX  LIST   OF   ILLUSTRATIONS 


1 06.  Effect  of  step  faults  in  repeating  outcrops 250 

107.  Model  showing  effect  of  dip  fault  on  outcrop  .....  251 

108.  Great  thrust  fault  near  Highgate  Springs,  Vt.  ....  252 

109.  Erosion  and  break  thrust,  Holly  Creek,  Ga.     .....  253 

no.  Great  thrust  fault  near  Highgate  Springs,  Vt.           ....  254 

in.  Model  showing  the  slip  of  folded  beds  upon  one  another         .         .  257 

112.  Model  showing  effects  of  lateral  compression  .         .         .         .         .  257 

113.  Fissile  quartzite,  California 260 

114.  Diagram  showing  relation  of  cleavage  and  stratiiication  planes        .  261 

115.  Slip  Rock,  Juniata  River,  Pennsylvania  ......  263 

116.  Dikes  of  sandstone  in  shales,  northern  California     ....  268 

117.  Unconformity:    diagrammatic  section  through   the  strata  seen  in 

Fig.  133 270 

118.  Unconformity  without  change  of  dip,  and  overlap  ....  271 

119.  Contemporaneous  erosion  in  limestone,  Iowa  .         ....  272 

120.  Volcanic  neck,  New  Mexico  ........  275 

121.  Jointed  lava  flow,  Passaic  River,  New  Jersey  .....  276 

122.  Diagram  of  dike 278 

123.  Dike  of  basalt  cutting  strata;    bad  lands  of  eastern  Oregon     .         .  279 

124.  Sheet  of  jointed  diabase;   Orange,  N.J.   ......  280 

125.  Palisades  of  the  Hudson,  New  Jersey      ......  281 

126.  Contact  of  intrusive  sheet  of  diabase  with  shales,  Wiehawken,  N.J.  282 

127.  Diagram  of  uneroded  laccolith        .......  283 

128.  Little  Sun  Dance  Hill,  South  Dakota 284 

129.  Mato  Tepee,  South  Dakota 285 

130.  Plicated  gneiss 296 

131.  Christiania  Fjord,  Norway       ........  307 

132.  Hog-back,  near  Golden,  Col.  .         .         .         .         .         .         -315 

133.  Mesa  and  round-topped  buttes,  bad  lands  of  South  Dakota     .         .  317 

134.  Evolution  of  a  river  system,  first  stage     ......  329 

135.  Evolution  of  a  river  system,  second  stage         .....  329 

136.  The  Charleston  Mountains,  Nevada,  one  of  the  Basin  Ranges          .  337 

137.  Ordovician  Coral,  Favistella  stettata 380 

138.  Triarthrus  Becki,  restoration 381 

139.  Silurian  fossils 390 

140.  Pterichthys  testudinarius          . 404 

141.  Cladoselache  Fyleri .  404 

142.  Dipterus  valenciennesi    .         .         .         .         ...         .         .  405 

143.  Coccosteus  decipiens       .         ....,»         .         .         .         .  406 

144.  Holoptychius  Andersoni          .         ,         .         ...         .         .  406 

145.  Pleuracanthus  Decheni  .         .        ,.    •    ,         «•-      *  434 


LIST  OK   ILLUSTRATIONS  xxi 

FIG.  PAGE 

146.  Eryops  megacephalus       .........  435 

147.  Glossopteris  inclica           .........  437 

148.  Voltzia  heterophylla 449 

149.  Diplurus  longicaudatus 453 

150.  Skull  of  Belodon  Kapjfi 454 

151.  Slab  of  Trigonia  clavellata,  from  the  English  Jura          .         .         .  463 

152.  Dapedius  politus 468 

153.  Aspidorhynchus  acutirostris    ........  468 

154.  Hypsocormus  insignis 468 

155.  Restoration  of  Ichthyosaurus  quadriscissus 469 

156.  Plesiosaurus  macrocephalus     ........  470 

157.  Restoration  of  Pterosaurian,  Khamphorhynchus       ....  472 

158.  Restoration  of  Archaopteryx  macrura 473 

159.  Cretaceous  leaves,  Dakota  stage 485 

1 60.  Sassafras  dissectum 486 

161.  Cinnamomum  affine 487 

162.  Clidastes  velox 490 

163.  Skull  of  Agathaumas  flabellatus 492 

164.  Skull  of  Diclonius  mirabilis   ........  492 

165.  Flabellaria  eocenica 502 

1 66.  Skeleton  of  Mesohippus  Bairdi 509 

167.  Skeleton  of  Hyracodon  nebrascense 510 

1 68.  Skeleton  of  Aphelops  fossiger 517 

169.  Skeleton  of  Toxodon  plalense 523 


EXPLANATION   OF   THE   PLATES 

PLATE  I.    AMERICAN  CAMBRIAN  FOSSILS 373 

Fig.    i.     Lingulella  coelata  Hall,  f .     Cambrian,  New  York  :    after 

Walcott. 
Fig.    2.     Agnostus  interstrictus  White,  f .    Middle  Cambrian,  Utah : 

after  Walcott. 
Fig.    3.     Conocoryphe  Kingi  Meek,   \.      Cambrian,   Utah:    after 

Meek. 
Fig.    4.     Elliptocephalus  Thompsoni  Hall,  \.     Upper  Cambrian, 

Vermont :  after  Walcott. 
Fig.    5.     Olenoidestypicalis  Walcott,  f    Middle  Cambrian,  Nevada : 

after  Walcott. 


xxii  LIST   OF   ILLUSTRATIONS 

PAGE 

PLATE  II.    AMERICAN  ORDOVICIAN  FOSSILS 383 

Fig.    I.     Brachiospongia    digitata    Owen,     \.        Trenton    Stage, 

Kentucky. 
Fig.    2.     Dicranograptus  ramosus  Hall,  ^.    Utica  Stage,  New  York : 

after  Hall. 
Fig.    3.     Diplograptus   pristis    Hiesinger,  T.       Utica   Stage,  New 

York  :  after  Ruedemann. 

Fig.    4.     Phyllograptus  typus  Hall,    }-.      Calciferous   Stage,  Can- 
ada, after  Hall. 
Fig.    5.     Dendrocrinus  polydactylus  Shumard,  \.     Hudson  Stage, 

Indiana:   after  Meek. 
Fig.    6.     Agelacrinus  cincinnatiensis  Roemer,  f.     Hudson  Stage, 

Ohio :  after  Meek. 

Fig.    7.     Orthis  lynx  Eichwald,  T.    Hudson  Stage,  Ohio:  after  Meek. 
Fig.    8.     Rhynchonella  capax  Conrad,  T.     Hudson  Stage,  Ohio: 

after  Meek. 
Fig.    9.     Strophomena  alternata  Conrad,  f.     Hudson  Stage,  Ohio: 

after  Meek. 
Fig.  10.     Ambonychia  radiata  Hall,  |.     Hudson  Stage,  Ohio  :  after 

Hall  and  Whitfield. 
Fig.  II.     Murchisonia  Milleri  Hall,  f.     Hudson  Stage,  New  York: 

after  Hall. 
Fig.  12.     Orthoceras  Duseri  Hall  and  Whitfield,  f.     Hudson  Stage, 

Ohio  :  after  Hall  and  Whitfield. 
Fig.  13.     Asaphus  gigas   Dekay,  \.     Hudson   Stage,  New  York  : 

after  Hall. 
Fig.  14.     Calymene  callicephala  Green,  f .     Hudson  Stage,  Ohio : 

after  Meek. 
Fig.  15.     Triarthrus   Becki   Green,   f.     Utica   Stage,  New  York: 

after  Hall. 
Fig.  1 6.     Trinucleus  concentricus  Eaton,  T.     Hudson  Stage,  New 

York :  after  Hall. 

Fig.  17.     Leperditia  fabulites  Conrad,  T.    Trenton  Stage,  Minne- 
sota :  after  Ulrich. 

PLATE  III.    AMERICAN  SILURIAN  FOSSILS 392 

Fig.    i.     Astylospongia   prsemorsa   Goldfuss,   T.      Niagara   Stage, 

Tennessee :  after  Roemer. 
Fig.    2.     Graptolithes  clintonensis   Hall,  T.     Clinton  Stage,  New 

York :  after  Hall. 
Fig.    3.     Favosites  Forbesi  E.  &  H.,  T.     Niagara  Stage,  Indiana: 

after  Hall. 


LIST  OF  ILLUSTRATIONS  xxiii 

PACE 

Fig.    4.     LepadocrinusGebhardi  Hall,  i.  Lower  Helderberg  Series, 

New  York :  after  Hall. 
Fig.    5.     Lingulella  cuneata  Conrad,  \.     Medina  Stage,  New  York  : 

after  Hall. 
Fig.    6.     Orthis  elegantula  Dalman,  ]-.     Niagara  Stage,  New  York  : 

after  Hall. 
Fig.    7.     Spirifera  crispa  Hiesinger,  ]•.     Niagara  Stage,  New  York  : 

after  Hall. 
Fig.    8.     Rhynchotreta  cuneata,  ] .     Niagara  Stage,  Indiana :  after 

Hall. 
Fig.    9.     Platyostoma  niagarense  Hall,  }.     Niagara  Stage,  Indiana : 

after  Hall. 
Fig.  10.     Cyclonema  cancellata  Hall,  ].     Clinton  Stage,  New  York  : 

after  Hall. 
Fig.  ii.     Capulus  angulatus  Hall,  j.     Niagara  Stage,  New  York: 

after  Hall. 
Fig.  12.     Orthoceras  annulatum  Sowerby,  \.      Niagara  Stage,  New 

York  :  after  Hall. 
Fig.  13.     Phragmoceras  parvum  H.  &  W.,  \.     Niagara  Stage,  Ohio: 

after  Hall. 
Fig.  14.     Trochoceras  desplainense  M'Chesney,  \.    Niagara  Stage, 

Wisconsin:    after  Hall. 

PLATE  IV.    AMERICAN  DEVONIAN  FOSSILS 401 

Fig.  i.  Heliophvllum  Halli  E.  &  H.,  £.  Hamilton  Stage,  Michi- 
gan :  after  Rominger. 

Fig.  2.  Acervularia  Davidsoni  E.  &  H.,  \.  Hamilton  Stage, 
Michigan :  after  Rominger. 

Fig.  3.  Spirifera  pennata  Atwater,  f.  Hamilton  Stage,  New 
York :  after  Hall. 

Fig.  4.  Athyris  spiriferoides  Eaton,  |.  Hamilton  Stage,  New 
York :  after  Hall. 

Fig.  5.  Rhynchonella  contracta  Hall,  f.  Chemung  Stage,  New 
York:  after  Hall. 

Fig.  6.  Pterinea  flabella  Conrad,  J.  Hamilton  Stage,  New  York  : 
after  Hall. 

Fig.  7.  Conocardium  trigonale  Hall,  f.  Corniferous  Stage,  Ohio  : 
after  Hall. 

Fig.  8.  Euomphalus  Decervi  Billings,  \.  Corniferous  Stage,  Ohio : 
after  Hall. 

Fig.  9.  Gomphoceras  mitra  Hall,  £.  Corniferous  Stage,  Indi- 
ana: after  Hall. 


XXIV  LIST  OF   ILLUSTRATIONS 

PAGE 

Fig.  10.     Goniatites  Vanuxemi   Hall,  ^.      Marcellus   Stage,    New 

York :  after  Hall. 
Fig.  II.  Homalonotus  Dekayi  Green,  ^.  Hamilton  Stage:  New 

York :  after  Hall. 
Fig.  12.  Phacops  rana  Hall,  |.  Hamilton  Stage,  New  York: 

after  Hall. 

PLATE  V.    AMERICAN  LOWER  CARBONIFEROUS  FOSSILS        .        .        .419 

Fig.  I.  Lithostrotion  canadense  Castelnau,  |.  St.  Louis  Stage, 
Iowa :  after  Hall. 

Fig.  2.  Pentremites  pyriformis  Say,  ^.  Chester  Stage,  Illinois: 
after  Hall. 

Fig.  3.  Productus  burlingtonensis  Hall,  -|.  Burlington  Substage, 
Iowa :  after  Hall. 

Fig.  4.  Chonetes  Fischeri  Norwood  &  Pratten,  •}.  Waverley  Stage, 
Iowa :  after  Hall. 

Fig.  5.  Spirifera  plena  Hall,  -|.  Burlington  Substage,  Iowa  :  after 
Hall. 

Fig.  6.  Onychocrinus  exsculptus  Lyon  &  Cassiday,  |.  Keokuk 
Substage,  Indiana :  after  Hall. 

Fig.  7.  Melonites  multipora  Owen  &  Norwood,  |.  St.  Louis 
Stage,  Missouri :  after  Roemer. 

Fig.  8.  Archimedes  Wortheni  Hall,  £.  Warsaw  Substage,  Illi- 
nois :  after  Hall. 

Fig.  9.  Platyceras  infundibulum  Meek  &  Worthen,  f .  Keokuk 
Substage,  Illinois :  after  Meek  &  Worthen. 

Fig.  10.  Bellerophon  sublsevis  Hall,  j.  Warsaw  Substage,  Indi- 
ana: after  Hall. 

Fig.  II.  Goniatites  ixion  Hall,  i.  Waverley  Stage,  Indiana :  after 
Hall. 

Fig.  12.  Conularia  missouriensis  Swallow,  ^.  St.  Louis  Stage, 
Missouri :  after  White. 

Fig.  13.  Phillipsia  bufo  Meek  &  Worthen,  f.  Keokuk  Substage, 
Indiana  :  after  Meek  &  Worthen. 

PLATE  VI.    AMERICAN  UPPER  CARBONIFEROUS  FOSSILS        .        .        .    423 

Fig.     I.     Fusulina  ventricosa  Meek  &  Hayden,  f.     Illinois:  after 

Meek  &  Hayden. 

Fig.    2.     ^siocrinus  magnificus  Miller  &  Gurley,  \.     Missouri. 
Fig.    3.     Spirifera  camerata  Morton,  f .     Iowa :  after  Hall. 
Fig.    4.     Productus  punctatus  Martin,  |.     Indiana:  after  White. 


LIST   OF  ILLUSTRATIONS 


XXV 


Fig.    5.     Euomphalus  subrugosus  Meek  &  Worthen,  -J-.     Illinois: 

after  Meek. 

Fig.    6.     Pleurotomaria  tabulata  Hall,  \.     Indiana  :  after  White. 
Fig.    7.     Loxonema  semicostata  Meek,  |.     Illinois :  after  Meek. 
Fig.    8.     Aviculopecten  neglectus  Geinitz,  -J-.     Illinois :  after  Meek. 
Fig.    9.     Allorisma   subcuneatum    Meek  &  Hayden,   \.     Indiana : 

after  White. 

P'ig.  10.     Sphenophyllum  Schlotheimi  Brogniart,  \.     Pennsylvania. 
Fig.  ii.     Pecopteris  orcopteridis  Schlotheim,  \.     Pennsylvania. 
Fig.  12.     Lepidoclendron    cuneatum    Lesquereux,    J    (fragment   of 

bark)  :  after  Rogers. 
Fig.  13.     Calamites   Suckowi   Brogniart,   \.      Pennsylvania:    after 

Lesquereux. 
Fig.  14.     Lophophyllum  proliferum  M'Chesney,  }.     Illinois :  after 

Meek. 

PLATE  VII.    AMERICAN  PERMIAN  FOSSILS 433 

Fig.    i.     Aviculopecten  occidentalis  Shumard,  \.     Kansas:    after 
C.  A.  White. 


Fig. 
Fig. 

Fig. 
Fig. 


Myalina  permiana  Swallow,  f .    Kansas :  after  C.  A.  White. 
Nautilus  Winslowi  Meek  &  Worthen,  {.     Texas:   after 

C.  A.  White. 
Medllcottia  Copei  White,  \.     Texas  :  after  C.  A.  White. 


Callipteris  conferta  Brogniart,  \.     Upper  Barren   Meas- 
ures, West  Virginia :  after  Fontaine  and  I.  C.  White. 
Fig.    6.     Sphenopteris  coriacea  F.  &  W.,  |.     Upper  Barren  Meas- 
ures, West  Virginia :  after  Fontaine  and  I.  C.  White. 


PLATE  VIII. 
Fig.  i. 
Fig. 
Fig. 
Fig. 
Fig. 


AMERICAN  TRIASSIC  FOSSILS 

Monotis  subcircularis  Gabb,  }.     California:  after  Gabb. 
Myophoria  alta  Gabb,  \.     Nevada:  after  Gabb. 
Trachyceras  Whitneyi  Gabb,  \.    Nevada  :  after  Gabb. 
Arcestes  Gabbi  Meek,  \.     Nevada  :  after  Gabb. 
Clathroptcris  platyphylla   Brogniart,  -J.     Newark   Series, 

New  Jersey :  after  Newberry. 

Otozamites  latior  Saporta,  \.    Newark  Series,  Connecti- 
cut :  after  Newberry. 

PLATE  IX.    AMERICAN  JURASSIC  FOSSILS 

Fig.     I.     Pentacrinus  asteriscus  Meek  &  Hayden,  fragment  of  stem, 

f .     South  Dakota :  after  Meek  &  Hayden. 
Fig.    2.     Gryphaea    nebrascensis     Meek    &    Hayden,    ^.       South 

Pakota :  after  Meek  &  Haydenr 


451 


Fig.    6. 


463 


xxvi  LIST  OF  ILLUSTRATIONS 


Fig.    3-     Quenstedioceras  cordiforme  Meek  &  Hayden,  f .     South 

Dakota :  after  Meek  &  Hayden. 
Fig.    4.     Belemnites  densus  Meek  &  Hayden,  f .     South  Dakota : 

after  Meek  &  Hayden. 
Fig.    5.     Trigonia  americana  Meek,  ^.     Montana:  after  Meek. 

PLATE  X.    AMERICAN  CRETACEOUS  FOSSILS 487 

Fig.    i.     Uintacrinus   socialis    Grinnell,   \.       Niobrara    Substage, 

Utah :  after  Clark. 
Fig.    2.     Pseudodiadema  texanum  Roemer,  \.     Comanche  Series, 

Texas :  after  Clark. 
Fig.    3.     Toxaster  texanus  Roemer,  \.     Comanche  Series,  Texas: 

after  Conrad. 
Fig.    4.     Terebratula  Harlani  Morton,  |.     Rancocas  Stage,   New 

Jersey :  after  Whitfield. 

Fig.    5.     Terebratella  plicata  Say,  \.    Navesink  Stage,  New  Jer- 
sey :  after  Whitfield. 
Fig.    6.     Ostrea  larva  Lamarck,  ^.     Navesink  to  Manasquan,  New 

Jersey :  after  Whitfield. 

Fig.    7.     Inoceramus  problematicus  Schlotheim,  f.     Niobrara  Sub- 
stage  :  after  Meek. 
Fig.    8.     Caprotina  bicornis  Meek,  ^.     Fox  Hills  Substage,  North 

Dakota :  after  Meek. 
Fig.    9.     Fasciolaria  buccinoides  Meek  &  Hayden,  -J-.     Fox  Hills 

Substage,  North  Dakota :  after  Meek. 
Fig.  10.     Anchura  americana   Evans  &   Shumard,  \.      Fox  Hills 

Substage,  North  Dakota :  after  Meek. 
Fig.  ii.     Margarita  nebrascensis  Meek  &  Hayden,  \.      Fox  Hills 

Substage,  North  Dakota :  after  Meek. 
Fig.  12.     Ptychoceras  Mortoni  Meek  &  Hayden,  f.      Fort  Pierre 

Substage,  North  Dakota :  after  Meek. 
Fig.  13.     Scaphites  nodosus  Owen,  ^.     Fort  Pierre  Substage,  North 

Dakota :  after  Meek. 
Fig.  14.     Baculites  compressus  Say,  |.    Fort  Pierre  Substage,  North 

Dakota :  after  Meek. 
Fig.  15.     Belemnitella  americana  Morton,  |.     Navesink  Stage,  New 

Jersey :  after  Whitfield. 

Fig.  1 6.     Nodosaria  texana  Conrad,  enlarged.    Texas :  after  Conrad. 
Fig.  17.     Micrabacia    americana   Meek  &  Hayden,  f.     Fox  Hills 

Substage,  North  Dakota  :  after  Meek. 
Fig.  1 8.     Aucella  Piochi  Gabb,  {.    Shasta  Series,  California :  after 

Gabb. 


LIST  OF  ILLUSTRATIONS  xxvii 

PAGE 

PLATE  XL    AMERICAN  TERTIARY  FOSSILS 504 

Fig.    i.     Ostrea  virginiana  Gmelin,  \.     Miocene,  New  Jersey  :  after 

Whitfield. 
Fig.    2.     Pecten  madisonicus  Say,  \.     Miocene,  New  Jersey :    after 

Whitfield. 
Fig.    3.     Cardita   perantiqua   Conrad,   }.      Eocene,   New  Jersey : 

after  Whitfield. 
Fig.    4.     Volutolithes  sayana  Conrad,  £.      Eocene,    New   Jersey: 

after  Whitfield. 
Fig.    5.     Oliva    carolinensis   Conrad,    -|.      Miocene,   New   Jersey: 

after  Whitfield. 
Fig.    6.     Helix  Dalli  Stearns,  \.     John  Day  Stage,  Oregon :  after 

White. 

Fig.    7.     Planorbis  convolutus  Meek  &  Hayden,  \.     Laramie  Cre- 
taceous, Montana:  after  Meek. 
P'ig.    8.     Aturia  Vanuxemi  Conrad,  £.      Eocene,  New  Jersey :  after 

Whitfield. 
Fig.    9.     Glyptostrobus   Ungeri    Heer,   \.     Green    River   Eocene, 

Wyoming:  after  Lesquereux. 
Fig.  10.     Salix  sp.,  £ .     Oligocene  of  Florissant,  Colorado. 

PLATE  XII.    TERTIARY  FOSSILS  FROM  FLORIDA 521 

Fig.    i.     Marginella  aurora  Ball,  J.     Miocene,  Florida:  after  Dall. 
Fig.    2.     Nassa  bidentata  Emmons,  |.    Miocene  &  Pliocene,  Florida : 

after  Dall. 
Fig.    3.     Murex  Conradi  Dall,  |.     Miocene,  South  Carolina :  after 

Dall. 

Fig.    4.     Natica  floridana  Dall,  |.     Miocene,  Florida :  after  Dall. 
Fig.    5.     Mitra  Willooxi  Dall,  £.     Pliocene,  Florida :  after  Dall. 
Fig.    6.     Fasciolaria  tulipa  Linn,  |.    Pliocene  to  Recent,  Florida : 

after  Dall. 

Fig.    7.     Typhis  floridanus  Dall,  }.      Pliocene,  Florida :  after  Dall. 
Fig.    8.    Turbo  rectogrammicus  Dall,  f .     Pliocene,  Florida :  after 

Dall. 


INTRODUCTION 


Geology  is  the  study  of  the  earths  history  and  development,  as  re- 
corded in  the  rocks,  and  of  the  agencies  which  have  produced  that 
development. 

From  this  definition  it  appears  that  the  central  problem  in  geol- 
ogy is  the  deciphering  of  the  earth's  history,  and  that  the  historical 
standpoint  is  the  dominant  one.  Geology  deals  with  the  earth  as 
a  cosmical  unit,  and  is  a  great  synthesis  of  all  those  sciences  which 
throw  light  upon  the  structure  of  the  globe  and  which  may  be  used 
in  interpreting  its  records.  Astronomy,  physics,  chemistry,  miner- 
alogy, physical  geography,  zoology,  and  botany  are  all  drawn  upon 
for  this  purpose.  The  goal  of  our  inquiries  is  the  history  of  the 
earth  as  a  whole,  and  not  of  a  single  continent  merely.  We  should 
endeavour  to  gain  a  true  insight  into  those  great  processes  of 
development  which  control  the  whole  visible  universe,  and  which 
exhibit  in  the  most  impressive  way  the  great  principles  of  order 
and  of  uniformly  acting  laws.  It  is,  however,  necessary  to  make 
a  selection  from  the  immense  body  of  ascertained  facts,  and  it  is 
clearly  advantageous  that,  so  far  as  possible,  we  should  make  use 
of  our  own  country  for  this  purpose.  It  must  always  be  remembered 
that  the  instances  chosen  from  familiar  scenes  are  but  illustrations 
and  examples  of  world-wide  processes  and  structures.  To  find 
active  examples  of  some  important  phenomena,  we  must  travel  to 
far-distant  lands,  but  even  of  these  we  shall  find  the  unmistakable 
traces  in  our  own  continent,  as  having  been  at  work  here  at  one 
time  or  another  in  the  past. 

Geology  is  one  of  the  most  modern  of  the  sciences.  In  the 
works  of  certain  classical  and,  mediaeval, writers  we  find,  it  is  true, 


2  INTRODUCTION 

some  descriptions  of  geological  phenomena,  and  sound  inferences 
were  sometimes  drawn  from  the  facts.  But  no  attempt  was  made 
to  gather  an  extensive  series  of  observations,  or  to  construct  a  har- 
monious system  of  facts  and  inferences,  and  no  one  imagined  that 
a  connected  history  of  the  earth  was  within  the  bounds  of  human 
attainment.  Before  such  a  history  could  be  written,  it  was  neces- 
sary that  the  other  physical  and  natural  sciences  should  have 
reached  a  considerable  degree  of  perfection ;  for  geology,  as  we 
have  seen,  is  a  synthesis  of  these  sciences.  It  was  only  in  the  latter 
part  of  the  eighteenth  century,  that  these  other  branches  of  know- 
ledge had  so  far  been  perfected  that  they  could  offer  to  the  geolo- 
gist a  firm  foundation  upon  which  to  build  the  structure  of  his 
own  science.  The  early  workers  in  geology  hardly  attempted 
more  than  to  ascertain  the  materials  of  which  the  earth  is  com- 
posed and  the  way  in  which  those  materials  are  put  together.  In 
carrying  out  this  apparently  simple  task,  it  soon  became  evident 
that  the  present  condition  of  the  earth  is  the  outcome  of  a  long 
series  of  past  changes,  which  must  be  understood  if  we  would 
comprehend  the  earth's  structure  as  it  is  now.  The  past  must  be 
studied  in  the  light  of  the  present,  and  the  present  in  the  light  of 
the  past,  for  each  supplements  and  helps  to  explain  the  other. 

Such  a  conclusion  is  repugnant  to  our  instinctive  feeling,  that  the 
earth  is  stable  and  well-nigh  unchangeable,  a  feeling  which  finds 
expression  in  Bryant's  familiar  line  :  "  The  hills  rock-ribbed  and 
ancient  as  the  sun."  This  very  natural  belief  is  due  to  the  exceeding 
slowness  with  which  the  surface  features  of  the  globe  are  modified, 
so  that  in  the  brief  span  of  human  life  the  modifications  are  almost 
imperceptible.  Generations  of  men  live  and  die  in  the  same  spot, 
while  the  natural  features  of  hill  and  rock,  valley  and  plain,  seem 
to  remain  exactly  as  they  were.  When,  however,  attention  was  at 
last  directed  to  these  changes,  it  was  found  that  they  were  unceas- 
ing, and  were  especially  noticeable  in  lands  which,  like  the  coun- 
tries around  the  Mediterranean,  had  been  occupied  for  many 
centuries  by  civilized  men.  Occasionally,  too,  great  tempests  or 
earthquakes  or  volcanic  outbursts  produced  changes  which  could 
not  but  strike  the  mostjcajeless  observer, .  r 


HISTORY   OF  GEOLOGY  3 

When  once  the  fact  was  established  that  the  solid  globe  was 
subject  to  change,  men  looked  first  to  the  more  obvious  and 
violent  natural  forces  as  the  agents  of  this  change.  To  the  occa- 
sional destructive  fury  of  the  hurricane,  the  earthquake,  and  the 
volcano  was  attributed  far  greater  importance  than  to  the  ceaseless 
but  inconspicuous  work  of  the  rain  and  the  river.  Another  reason 
why  sudden  and  violent  catastrophes  were  regarded  as  the  only 
important  factors  of  change,  was  the  very  general  belief  that  the 
earth  was  only  a  few  thousand  years  old.  If  all  the  modifications 
which  the  earth's  surface  has  demonstrably  undergone  were  effected 
within  such  a  comparatively  brief  period,  then  they  must  have 
been  accomplished  suddenly  and  violently  and,  in  great  part,  by 
agencies  of  which  we  have  had  no  experience.  Thus,  all  sorts  of 
fantastic  causes,  such  as  collisions  with  comets'  tails,  were  conjured 
up  to  account  for  the  facts,  and  speculation  ran  riot. 

The  purely  arbitrary  character  of  these  speculations  and  fancies 
rendered  them  unsatisfactory  to  thinking  men.  The  progress  of 
astronomy  had  gradually  familiarized  their  minds  with  the  idea  of 
the  almost  infinite  distances  which  separate  the  heavenly  bodies, 
and  with  the  conceptions  of  order  and  the  uniform  operation  of 
law.  These  conceptions  made  the  supposed  cataclysms  and  con- 
vulsions of  the  earth's  history  seem  unnatural  and  improbable,  and 
led  geologists  to  inquire  whether  simpler  explanations  could  not 
be  devised.  This,  in  turn,  led  to  the  careful  study  of  those  modi- 
fications of  the  earth's  surface  which  are  still  in  progress.  Gradu- 
ally the  conviction  grew,  that  the  agencies  which  are  still  at  work 
upon  and  within  the  globe  are  the  same  as  those  which  brought 
about  the  manifold  changes  of  the  past,  and  that  the  earth's  his- 
tory is  one  of  vast  and  unimaginable  length.  Scholars  made  little 
progress  in  deciphering  the  Egyptian  hieroglyphics  until  the  dis- 
covery of  the  Rosetta  Stone,  with  its  bilingual  inscriptions,  fur- 
nished the  key.  So  the  geologists  found  that  one  key  to  the  past 
was  to  be  found  in  the  study  of  the  forces  which  may  be  observed 
in  actual  operation  at  the  present  time. 

Another  advance  was  made  while  the  disputes  regarding  the 
nature  of  geological  forces  and  the  length  of  geological  time  were 


4  INTRODUCTION 

still  in  progress.  In  1799  William  Smith,  an  English  engineer, 
announced  the  discovery  that  the  order  of  succession  of  the  rocks 
might  be  determined  by  means  of  the  fossils,  or  remains  of  ani 
mals  and  plants,  which  they  contain.  This  discovery  was  made 
possible  by  the  advances  in  zoology,  by  means  of  which  the  differ- 
ent species  could  be  accurately  discriminated.  Since  the  earliest 
recorded  times,  the  animals  and  plants  of  the  earth  have  been 
subject  to  continual  change,  and  the  degrees  of  change  give  us  a 
standard  of  chronology,  in  accordance  with  which  the  various 
groups  of  rocks  may  be  arranged  in  their  order  of  succession. 
The  archaeologist  makes  a  similar  use  of  the  coins,  inscriptions, 
and  objects  of  art,  which  he  finds  among  the  ruins  of  buried  cities. 
These  enable  him  to  determine  the  races  of  men  who  built  those 
cities  and  the  dates  at  which  they  flourished,  and  to  fix  their  place 
in  the  general  history  of  civilization. 

The  lines  along  which  geology  has  developed  were  nearly  all 
laid  down  late  in  the  last  century,  or  early  in  the  present  one,  but 
the  progress  of  the  science  has  led  to  many  changes  in  men's 
conceptions  of  the  subject,  some  of  which  changes  have  been 
revolutionary.  Geology  began  with  the  study  of  western  Europe, 
and  on  account  of  this  narrowly  restricted  range  of  view,  erroneous 
notions  naturally  crept  into  the  new  science.  As  the  study  has 
been  extended  to  other  continents,  new  and  larger  views  have 
been  gained,  and  doubtless,  when  the  whole  earth  has  been  accu- 
rately examined,  many  of  our  present  opinions  will  need  revision, 
though  we  cannot  hope  ever  to  reach  final  certainty  upon  all 
points. 

To  many  intelligent  people  this  continual  modification  of  scien- 
tific opinion,  which  is  a  necessary  consequence  of  advancing  know- 
ledge, is  a  source  of  annoyance.  This  attitude  of  mind  comes  from 
a  failure  to  discriminate  between  fact,  and  inference  or  hypothesis. 
Accurately  observed  facts  may  be  added  to,  but  they  remain  trust- 
worthy :  the  changeable  element  is  the  inference  which  is  drawn 
from  the  facts.  These  inferences  are  of  very  different  degrees  of 
certainty.  Some  such  deductions  which  were  made  centuries  ago 
remain  unshaken  to-day,  while  others  of  far  more  recent  date  have 


FACTS  AND   INFERENCES  5 

proved  illusory.  'Thus,  when  we  find  a  rock  composed  of  cemented 
sand-grains,  arranged  in  regular  beds  or  layers,  we  infer  that  it 
was  laid  down  under  water,  because  of  its  exact  resemblance  to 
accumulations  of  sand  which  are  forming  under  water  to-day.  If 
the  sandstone  be  full  of  marine  shells,  we  infer  that  it  was  formed 
under  the  sea,  and  further  that  the  land  where  the  rock  is  now 
found  was  once  covered  by  the  sea.  Such  inferences  are  prac- 
tically certain,  because  they  explain  all  the  known  facts  and  are 
in  conflict  with  none.  On  the  other  hand,  the  hypotheses  of 
Cuvier  and  others  as  to  the  character  of  the  earth's  development, 
and  the  manner  in  which  the  successive  assemblages  of  animals 
and  plants  were  called  into  being,  were  abandoned  long  ago/' 

In  the  process  of  reasoning  from  the  known  to  the  unknown, 
the  inferences  become  the  more  uncertain,  the  farther  we  recede 
from  demonstrable  facts.  Hypotheses  are  assumptions  which  we 
make  to  explain  and  coordinate  large  numbers  of  facts,  and  so 
long  as  their  true  nature  is  understood,  they  are  useful,  indeed 
indispensable,  means  of  reaching  the  truth.  The  objection  is  that 
they  are  too  often  taught  as  though  they  were  established  beyond 
dispute.  A  true  hypothesis  will  prove  to  be  in  harmony  with 
newly  discovered  facts,  which  will  take  their  place  under  it  simply 
and  naturally.  A  false  hypothesis,  on  the  other  hand,  may  be  in 
accordance  with  all  the  facts  known  at  the  time  when  it  was  pro- 
posed, but  the  progress  of  discovery  will  bring  to  light  facts  which 
are  inconsistent  with  the  hypothesis,  until  it  is  plainly  seen  to  be 
inadequate  and  misleading.  Yet  even  a  false  hypothesis  may  serve 
a  useful  purpose,  for  it  puts  before  us  a  definite  problem,  instead 
of  a  mere  catalogue  of  uncorrelated  facts.  The  pathway  of  every 
science  is  strewn  with  wrecks  of  hypotheses  which  have  been  used, 
worn  out,  and  thrown  aside.  In  all  our  thinking  and  reasoning 
the  distinction  between  hypothesis  and  fact  must  be  steadily  held 
in  view. 

While,  in  its  most  comprehensive  sense,  geology  consists  in  the 
application  of  nearly  all  the  physical  and  natural  sciences  to  the 
elucidation  of  the  earth's  history,  the  geologist  has  his  own  special 
field  of  investigation.  This  he  finds  in  the  rocks,  and  every  ex- 


6  INTRODUCTION 

posure  of  rocks  yields  him  material.  It  might  seem  that,  at  best, 
his  studies  must  be  very  superficial,  and  that  he  must  soon  eke 
out  his  scanty  facts  with  daring  guess-work.  Such  is  happily  not 
the  case.  The  fact  that  rocks  of  different  ages  were  formed  in 
different  places,  and  that  great  disturbances  have  so  tilted  immense 
bodies  of  rocks  that  their  edges  are  exposed  to  view,  enables  the 
observer  to  study  vast  thicknesses  of  them,  without  descending 
below  the  surface  of  the  ground.  Seventy-five  years  ago  Playfair 
saw  and  expressed  this  truth.  "  Men  can  see  much  further  into 
the  interior  of  the  globe  than  they  are  aware  of,  and  geologists  are 
reproached  without  reason  for  forming  theories  of  the  earth,  when 
all  they  can  do  is  but  to  make  a  few  scratches  on  the  surface." 

The  history  of  the  earth  has  been  recorded,  for  the  most  part, 
upon  its  successive  surfaces,  and  it  is  not  necessary  to  penetrate 
deep  into  the  interior  of  the  globe.  In  later  chapters  we  shall 
learn  how  these  surfaces  came  to  be  buried  to  great  depths  and 
yet  retained  the  characters  impressed  upon  them  when  they  were 
superficial  in  position,  as  written  pages  are  buried  under  fresh 
accumulations  of  manuscript.  This  fact,  together  with  the  dis- 
turbances which  have  made  the  deep-seated  rocks  accessible  to 
study,  renders  the  task  of  deciphering  the  record  less  hopeless 
than  might  be  imagined. 

The  study  of  geology  must  be  carried  on  at  first  hand,  and 
cannot  be  adequately  learned  from  books  alone.  The  use  of 
books  is  to  serve  as  guides  in  directing  the  learner  in  what  to 
look  for  and  to  enable  him  to  compare  distant  lands  with  his 
own.  The  arrangement  and  treatment  of  such  a  complex  subject 
as  geology  cannot  avoid  a  certain  artificial  character,  which  will 
surely  mislead  the  student,  unless  he  learns  to  observe  and  reason 
for  himself.  Some  parts  of  our  country  are  more  favourable  to 
geological  study  than  others,  but  none  is  entirely  devoid  of  geo- 
logical interest,  and  the  operations  of  the  dynamical  agencies  may 
be  watched  everywhere.  To  one  who  thus  familiarizes  himself 
with  the  structure  and  history  of  the  country,  every  landscape 
will  offer  a  renewed  charm  and  interest.  The  study  of  the  rocks 
will  lead  him  step  by  step  to  the  widest  outlook  over  the  history 


DIVISIONS    OF  GEOLOGY  7 

of  the  earth,  to  the  contemplation  of  infinities  of  energy,  space, 
and  time,  bringing  him  at  last  face  to  face  with  the  mystery 
of  the  Universe.  To  this  inscrutable  mystery  every  line  of  scien- 
tific inquiry  must  ultimately  lead,  for  human  knowledge  is  like 
a  dim  taper  which  illumines  a  little  space,  but  is  surrounded  by 
immeasurable  darkness. 

The  very  diverse  lines  of  inquiry  which  together  make  up  the 
science  of  geology,  must  be  classified  and  divided  for  the  purposes 
of  orderly  treatment.  The  following  are  the  principal  divisions  of 
the  science. 

1.  Dynamical  Geology,  or  the  study  of  the  forces  which  are 
now  at  work  in  modifying  the  surface  of  the  earth,  and  of  the 
chemical  and  mechanical  changes  which  they  effect.     This  is  the 
key  by  which  we  may  interpret  past  changes. 

2.  Structural  Geology,  or  the  study  of  the  materials  of  which 
the  earth  is  composed  and  of  the  manner  in  which  they  are 
arranged  ;  together  with  such  explanations  of  the  modes  in  which 
the   arrangement  was   produced,  as  may  be   inferred   from   the 
structure. 

3.  Physiographical  Geology  is   an   examination  of  the   topo- 
graphical features  of  the   earth  and  of  the  mode  in  which  they 
were  produced.     Primarily,  this  subject  is  a  province  of  physical 
geography,  but  it  is  a  valuable  adjunct  to  geology. 

The  three  foregoing  divisions  together  constitute  a  larger  division, 
which  is  called  Physical  Geology,  and  which  is  contrasted  with  — 

4.  Historical  Geology.  —  This  is  the  study  of  the  earth's  history 
and  development,  the  changes  of  level  between  land  and  sea,  of 
topography,  of  climate,  and  of  the  successive  groups  of  animals 
and  plants  which  have  lived  upon  the  globe.     As  we  have  seen, 
the  historical  is  the  dominant  standpoint  in  geology,  the   main 
problem  of  which  is  to  interpret  the  records  of  the  earth's  history. 
The  other  departments  are  to  be  regarded  as  the  means  to  this 
great  end. 


CHAPTER   I 
THE  ROCK-FORMING  MINERALS 

OF  the  simple,  undecomposable  substances,  which  chemists  call 
elements,  and  of  which  somewhat  more  than  seventy  have  been 
identified  on  the  earth,  only  about  twenty  enter  at  all  largely  into 
the  composition  of  the  earth's  crust,1  so  far  as  this  is  accessible  to 
examination.  It  is  estimated  that  97  per  cent  of  the  crust  is  made 

up  of  ten  elements. 

» 

NONMETALLIC.  METALLIC. 

Oxygen O.  Aluminium      ....  Al. 

Hydrogen H.  Potassium K. 

Silicon Si.  Sodium Na. 

Carbon C.  Calcium Ca. 

Magnesium     ....  Mg. 

Iron Fe. 

The  remaining  ten  are  far  less  abundant,  but  yet  of  considerable 
importance. 

Chlorine Cl.  Lithium Li. 

Fluorine F.  Barium Ba. 

Sulphur S.  Manganese      ....  Mn. 

Phosphorus      ....  P.  Titanium Ti. 

Boron B.  Zirconium Zr. 

Only  two  of  these  elements,  carbon  and  sulphur,  are  found  in  a 
more  or  less  impure  state  as  minerals  or  rock  masses ;  the  others 

1  By  crust  of  the  earth  is  meant  its  exterior  portion  or  shell  of  indefinite  thick- 
ness. It  does  not  necessarily  imply  any  very  radical  difference  from  the  interior, 
though  the  term  was  first  employed  to  denote  the  solid  external  layer  which  covered 
a  supposed  liquid  interior. 

8 


CRYSTALLINE   FORMS 


occur  as  compounds,  formed  by  the  union  of  two  or  more  of 
them. 

A  mineral  is  a  natural,  inorganic  substance,  which  has  a  homo- 
geneous structure,  definite  chemical  composition  and  physical 
properties,  and  usually  a  definite  crystalline  form. 

Crystals  are  solids  of  more  or  less  regular  and  symmetrical  shape, 
bounded,  usually,  by  plane  surfaces.  The  number  of  known  crys- 
talline forms  is  already  very  great,  and  yet  they  may  be  all  reduced 
to  thirteen  fundamental  shapes,  which  are  prisms,  octahedrons 
(eight-sided),  or  dodecahedrons  (twelve-sided). 

The  thirteen  fundamental  forms  and  their  innumerable  secondary 
derivatives  fall  into  six  systems,  which  are  characterized  by  the  re- 
lations of  their  axes.  The  axes  of  a  crystal  are  imaginary  lines, 
which  connect  the  centres  of  opposite  faces,  or  opposite  edges,  or 
opposite  solid  angles,  and  which  intersect  one  another  at  a  point  in 
the  interior  of  the  crystal. 

The  Systems  of  Crystalline  Forms  have  received  many  names, 
the  following  being  those  which  are  most  generally  used  in  this 
country :  — 

I.  Isometric  System  (monometric,  cubical,  regular).  —  In  this 
system  the  three  axes  are  of  equal 

length  and  intersect  one  another 
at  right  angles ;  it  includes  the 
cube,  regular  octahedron,  and 
rhombic  dodecahedron,  forms 
which  are  symmetrical  in  all 
positions. 

II.  Tetragonal  System  (dimetric,  pyramidal) .  —  The  axes  inter- 

sect at  right  angles,  but  are  not 
all  of  equal  length ;  the  two 
lateral  axes  are  of  equal  length, 
but  the  vertical  axis  is  longer  or 
shorter  than  the  laterals.  In- 
FIG.  2.  — Forms  of  the  Tetragonal  eludes  the  right  square  prism  and 

System:    Right  Square  Prism;  Square    th  r£  octahedron,   the  faces 

Octanedron. 

of  which  are  isosceles  triangles. 


FIG.  i.  —  Forms  of  the  Isometric 
System:  Cube;  Regular  Octahedron; 
Rhombic  Dodecahedron. 


TO 


THE   ROCK-FORMING   MINERALS 


III.   Hexagonal  System.  —  Here  four  axes  are  employed,  three 

equal  lateral  axes  intersecting  at 
angles  of  60  degrees,  and  a  ver- 
tical axis,  which  is  perpendicular 
to  and  longer  or  shorter  than  the 
laterals.  Includes  the  rhom- 
bohedron,  hexagonal  prism,  and 
scalenohedron. 

IV.    Orthorhombic   System 


FIG.  3.  —  Forms  of  the  Hexagonal 
System  :  Hexagonal  Prism  ;  Rhombo- 
hedron ;  Scalenohedron. 


FIG.  4.  —  Forms  of  the  Orthorhom- 
bic System :  Rhombic  Octahedrons. 


(rhombic,  trimetric) .  —  The  three  axes  intersect  at  right  angles 
and  are  all  of  different  lengths ; 
rectangular  and  rhombic  prisms, 
and  rhombic  octahedron. 

V.  Monoclinic  System  (mono- 
symmetric,  oblique) .  —  All  three 
axes  are  of  different  lengths ;  two  of  the  axes,  usually  the  laterals, 

are  at  right  angles  to  each  other, 
while  the  third  is  oblique  :  right 
rhomboidal  and  oblique  rhom- 
bic prisms. 
VI.    Triclinic  System  (anor- 


FlG.  5.  —  Forms    of  the    Monoclinic 
System  :  Monoclinic  Pyramid  and  Prism. 


thic,       asymmetric).  —  Three 
axes   of  unequal    lengths   and 
oblique    rhomboidal    prism,    doubly 


oblique    to    one    another : 
oblique  octahedron. 

It  is  important  to  bear  in  mind  the  relations  which  the  funda- 
mental forms  sustain  toward  one  another.  For  example,  a  regular 
octahedron  may  be  derived  from  a  cube  by  evenly  paring  off  the 
eight  solid  angles,  until  the  planes  thus  produced  intersect  one 
another,  the  centres  of  the  faces  of  the  cube  becoming  the  apices 
of  the  solid  angles  of  the  octahedron.  Conversely,  a  cube  may  be 
formed  from  an  octahedron  by  symmetrically  truncating  the  angles, 
until  the  planes  thus  formed  intersect.  By  slicing  away  the  twelve 
edges  of  a  cube  or  an  octahedron  a  dodecahedron  will  result. 
These  crystalline  forms  are,  therefore,  so  related  as  to  be  all  de- 
rivable one  from  another,  and  the  relations  of  their  axes  remain 


PHYSICAL   PROPERTIES  OF  MINERALS  II 

unchanged  ;  all  three  forms  may  be  assumed  by  the  same  mineral, 
and  they  thus  properly  belong  in  the  same  system.  Similar  relations 
may  be  observed  between  the  crystalline  forms  of  the  other  systems. 

It  might  be  supposed  that  the  crystalline  systems  and  the  rela- 
tions of  their  imaginary  axes  were  merely  mathematical  devices  to 
reach  a  convenient  classification  of  forms.  Such  a  conclusion 
would,  however,  be  a  very  erroneous  one.  Crystalline  form  is  the 
expression  of  molecular  structure,  and  many  of  the  physical  proper- 
ties of  minerals  are  determined  by  their  mathematical  figure.  It 
is  clear  that  the  physical  properties  which  depend  upon  form  are 
not  inherent  in  the  molecules  of  the  mineral,  but  are  conditioned 
by  the  way  in  which  the  molecules  are  built  up  into  the  crystal. 
Amorphous  substances  refract  light  equally  in  all  directions,  and 
are  thus  called  iso tropic  ;  but  when  an  amorphous  substance  crystal- 
lizes, it  assumes  the  qualities  proper  to  its  crystalline  form.  Thus 
water  is  isotropic,  while  the  hexagonal  crystals  of  ice  are  singly 
refractive  in  only  one  direction,  doubly  refractive  in  two.  The 
same  substance  may,  under  different  circumstances,  crystallize  in 
different  systems,  and  will  then  display  the  properties  appropriate 
to  each  system. 

Not  only  the  refractive  powers  of  a  crystal,  but  also  its  mode  of 
expansion  when  heated,  and  its  conductivity  of  electricity  and  heat 
depend  upon  its  form. 

The  crystals  of  the  isometric  system,  which  have  their  three  axes 
of  equal  length,  are  singly  refractive  in  all  directions,  expand 
equally  when  heated,  and  conduct  heat  and  electricity  equally  in 
all  directions.  Those  of  the  tetragonal  and  hexagonal  systems, 
which  have  one  axis  longer  or  shorter  than  the  others,  are  doubly 
refractive  along  the  lateral  axes,  expand  equally  when  heated,  and 
show  equal  conductivity  along  these  axes.  Along  the  principal 
axis  they  are  singly  refractive,  expand  to  a  different  degree  when 
heated,  and  display  a  different  conductivity  along  this  axis  than 
along  the  others.  In  the  orthorhombic,  monoclinic,  and  triclinic 
systems,  which  have  all  the  axes  of  unequal  lengths,  the  crystals 
are  singly  refractive  in  two  directions ;  they  expand  unequally  and 
conduct  differently  along  all  their  axes, 


12  THE   ROCK-FORMING   MINERALS 

The  optical  properties  of  minerals  are  of  great  value  in  the 
study  of  rocks,  and  by  the  aid  of  the  polarizing  microscope  very 
minute  crystals  may  be  identified. 

Most  substances  which  are  solid  under  any  circumstances  are 
capable  of  assuming  a  crystalline  form,  so  that  solidification  and 
crystallization  are  usually  identical.  For  the  formation  of  large 
and  regular  crystals,  it  is  necessary  that  the  process  be  gradual  and 
that  space  be  given  for  the  individual  crystals  to  grow.  Usually 
crystallization  begins  at  many  points  simultaneously,  and  the  crys- 
tals crowd  upon  one  another,  resulting  in  a  mass  of  more  or  less 
irregular  crystalline  grains.  The  same  substance  which,  when  very 
rapidly  solidified,  forms  an  amorphous  glass,  will  give  rise  to  dis- 
tinct crystals,  if  slowly  solidified. 

Crystallization  requires  that  the  molecules  be  free  to  move  upon 
each  other,  and  thus  to  arrange  themselves  in  a  definite  fashion. 
It  may  take  place  either  by  the  deposition  of  a  solid  from  solu- 
tion, by  cooling  from  a  state  of  fusion,  or  by  solidification  from 
the  condition  of  vapour.  In  all  cases  the  size  and  regularity 
of  the  crystals  depend  upon  the  time  and  space  allowed  for 
their  growth.  In  a  manner  not  yet  understood,  amorphous 
solids  may  be  converted  into  crystalline  aggregates.  This  has 
been  observed  in  the  case  of  certain  glassy  volcanic  rocks,  which, 
though  amorphous  when  first  solidified,  have  gradually  become 
crystalline,  without  losing  their  solidity.  This  process  is  called 
devitrification. 

The  actual  steps  of  crystallization  may  be  observed  by  slowly 
evaporating  a  solution  of  some  crystalline  salt  under  the  micro- 
scope. The  first  visible  step  in  the  process  is  the  appearance  of 
innumerable  dark  points  in  the  fluid,  which  rapidly  grow,  until 
their  spherical  shape  is  made  apparent.  The  globules  then  begin 
to  move  about  rapidly  and  arrange  themselves  in  straight  lines, 
like  strings  of  beads,  and  next  suddenly  coalesce  into  straight  rods. 
The  rods  arrange  themselves  into  layers,  and  thus  build  up  the 
crystals  so  rapidly,  that  it  is  hardly  possible  to  follow  the  steps  of 
change.  In  certain  glassy  rocks,  which  solidified  too  quickly  to 
allow  crystallization  to  take  place,  the  incipient  stages  of  crystals, 


PROPERTIES  OF   MINERALS  13 

in  the  form  of  globules  and  hair-like  rods,  may  be  detected  with 
the  microscope. 

Secondary  Forms  of  Crystals.  —  A  great  variety  of  crystalline 
forms  is  produced  by  the  occurrence  of  secondary  planes  or  faces 
on  the  angles  or  edges  of  the  primary  forms.  All  the  similar  parts 
of  the  crystal  may  be  modified  in  the  same  way,  or  alternating 
similar  parts  may  be  so  modified. 

Certain  faces  may  be  obliterated  by  the  enlargement  of  others ; 
but  however  great  the  variation,  the  angle  at  which  corresponding 
faces  meet  almost  invariably  remains  constant  for  each  mineral. 

Massive  and  imperfectly  crystallized  minerals  may  consist  of 
grains,  fibres,  or  thin  layers  (lamina). 

Hardness.  —  The  hardness  of  minerals  is  a  useful  means  of  iden- 
tifying them.  For  this  purpose  they  are  referred  to  a  scale  of 
hardness,  ranging  from  such  soft  substances  as  may  be  readily 
scratched  with  the  finger-nail,  to  the  hardest  known  substance, 
diamond.  The  degree  of  hardness  is  expressed  by  the  numerical 
place  of  the  mineral  in  the  scale,  and  intermediate  grades  are 
indicated  by  fractions.  Thus  a  mineral  which  is  scratched  by 
quartz  and  scratches  orthoclase  with  equal  ease,  has  a  hardness  of 
6.5.  The  scale  is  as  follows  :  — 

1.  Talc.  6.  Orthoclase. 

2.  Selenite.  7.  Quartz. 

3.  Calcite.  8.  Topaz. 

4.  Fluor-spar.  9.  Sapphire. 

5.  Apatite.  10.  Diamond. 

Cleavage.  —  Many  minerals  split  readily  along  certain  planes, 
still  retaining  a  crystalline  form,  while  in  other  directions  they 
break  irregularly.  This  property  is  called  cleavage.  Cleavage 
is  uniform  in  the  different  varieties  of  the  same  mineral,  and 
takes  place  either  in  planes  parallel  to  one  or  more  faces  of  the 
fundamental  form  of  the  crystal,  or  along  the  diagonals  of  that 
form. 

Pseudomorphs  occur  when  one  mineral  assumes  the  crystalline 
form  proper  to  another.  This  may  take  place  either  by  the  addi- 


14  THE   ROCK-FORMING   MINERALS 

tion  or  the  removal  of  certain  constituents,  or  some  constituents 
may  be  removed  and  others  substituted  for  them.  The  entire 
substance  of  a  mineral  may  be  removed  and  its  place  taken, 
molecule  by  molecule,  by  another,  retaining  the  form,  sometimes 
even  the  cleavage,  of  the  first.  The  study  of  pseudomorphs  is 
often  of  the  greatest  service,  as  throwing  light  upon  the  history 
of  the  rock  in  which  they  occur. 

Compound  crystals  are  formed  by  the  joining  of  simple  crystals. 
When  two  half- crystals  are  united  along  a  plane  in  such  a  way 
that  their  faces  and  axes  do  not  correspond,  they  are  said  to  be 
twinned.  When  the  twinning  is  repeated  along  numerous  parallel 
planes,  the  crystal  is  a  polysynthetic  fruin.  Two  crystals  united  at 
the  ends  to  form  a  right  angle,  are  called  geniculafe,  while  two 
geniculate  crystals  may  be  so  combined  as  to  form  a  cross,  and 
then  are  said  to  be  cruciform. 

Crystals  of  the  same  form  may  vary  in  length  and  in  the  size  of 
their  corresponding  faces,  which  gives  rise  to  numerous  irregulari- 
ties of  shape. 

Rock-forming  Minerals. — The  number  of  known  minerals  is 
exceedingly  great,  and  is  constantly  increasing,  but  only  a  few 
enter  in  any  important  way  into  the  constitution  of  the  earth's 
crust.  We  now  proceed  to  a  consideration  of  these  constituent 
minerals,  which  are  called  rock-forming  minerals,  because  the 
rocks  are  aggregations  of  them.  It  must  be  emphasized  that  the 
student  can  gain  no  real  knowledge  of  minerals  or  rocks  by  merely 
reading  about  them;  it  is  necessary  that  he  should  familiarize 
himself  with  actual  specimens. 

A.    MINERALS  COMPOSED  OF  SILICA 

Next  to  oxygen,  silicon  is  by  far  the  most  abundant  constituent 
of  the  earth's  crust,  though  never  occurring  alone.  It  is  united 
with  oxygen  to  form  silica  (SiO2)  or  enters  into  the  formation  of 
more  complex  compounds.  The  oxide,  silica,  is  the  commonest 
mode  of  occurrence  and  form?  the  most  abundant  of  all  the 
minerals, 


SILICA   MINERALS  15 

1.  Quartz  (Si(X)  is  anhydrous  silica  in  a  crystalline  state;  it 
belongs  in  the  hexagonal   system,  and  crystallizes  in  hexagonal 
prisms   capped   by   six-sided   pyramids,  or  in   double   six-sided 
pyramids,  or  in  modifications  of  these  forms.     It  is  insoluble  in 
any  acid  except  hydrofluoric,  and  only  very  slowly  soluble  in  boil- 
ing caustic  alkalies.     When  dissolved,  as  may  be  done  by  some- 
what complicated  processes,  silica  shows  a  distinct  acid  reaction. 

Quartz  has  no  cleavage  and  is  very  hard  (H=  7),  scratching 
glass  readily,  while  it  cannot  be  scratched  with  a  knife ;  the  spe- 
cific gravity  (sp.  gr.)  is  2.6. 

When  pure  and  symmetrically  crystallized,  quartz  is  transparent, 
colourless,  and  lustrous  (rock  crystal),  but  it  more  commonly  is 
found  in  dull  masses.  Different  varieties  are  coloured  by  metallic 
oxides  :  thus,  amethyst  is  quartz  stained  purple  by  the  oxide  of 
manganese  ;  smoky  quartz,  or  cairngorm,  owes  its  brownish  or  yel- 
.owish  colour  to  the  oxide  of  iron  ;  and  there  are  many  other  kinds. 

In  its  various  forms  quartz  is  the  most  abundant  of  minerals, 
and  plays  the  most  important  part  in  the  formation  of  the  different 
classes  of  rocks. 

2.  Chalcedony  occurs  in  spheroidal  or  stalactitic  masses,  com- 
posed of  more  or  less  concentric  shells.     The  structure  is  crystal- 
line, and  displays  radiating  fibres,  which  are  perpendicular  to  the 
shells.     The  chemical  composition  and  behaviour  of  this  mineral 
are  the  same  as  in  quartz,  but  the  specific  gravity  is  somewhat 
lower  (2.59-2.64),  and  the   optical  properties  different.     Chal- 
cedony has  a  waxy  appearance,  and  is  translucent  or  semi-opaque, 
and  of  various  pale  colours. 

3.  Opal,   Hyalite.      Hydrated   silica    (SiO2,   *H2O). — These 
minerals  are  amorphous  and  have  no  crystalline  form.     Opal  is 
either  translucent  or  opaque  and  of  various  colours.    Precious  opal 
(a  gem)  and  common  opal  differ  in  colour,  and  in  the  fact  that 
the  former  is  iridescent,  the   latter   not.      Hyalite   is   colourless 
and  transparent.     Hydrated  silica  is  lighter  than  the  anhydrous 
(sp.  gr.  =  2.2)   and  more  readily  soluble  in  hot  alkaline  waters. 
These  minerals  are  of  much  less  importance  as  constituents  of 
rock  than  the  forms  of  quartz. 


1 6  THE   ROCK-FORMING   MINERALS 

4.  Agate  is  a  banded  mineral,  composed  of  layers  of  amorphous 
and  crystalline  silica,  chalcedony,  jasper,  amethyst,  rock  crystal,  etc. 

5.  Flint  and  Chert  are  also  believed  to  be  mixtures  of  hydrated 
and  anhydrous  silica.     They  occur  in  amorphous  masses  of  neutral 
or  dark  colours,  and  are  opaque,  or  somewhat  translucent  in  thin 
pieces. 

B.    MINERALS  COMPOSED  OF  SILICATES 

Silica  is  an  acid  and  forms  a  very  extensive  series  of  compounds 
with  various  metallic  bases.  As  rock-forming  minerals  the  silicates 
are  second  only  to  the  silica  minerals  in  importance. 

I.  THE  FELSPAR  GROUP 

The  felspars  are  essentially  silicates  of  alumina  (A12O3)  to- 
gether with  potash,  soda,  or  lime.  Three  primary  felspars 
occur:  orthoclast,  a  potash  felspar  (K2O,  A12O3, 6SiO2)  ;  albite, 
a  soda  felspar  (Na2O,  A12O3,  6  SiO2)  ;  and  anorthite,  a  lime  felspar 
(2  CaO,  2  A12O3, 4  SiO2) .  From  the  combination  of  these  primary 
minerals  two  series  are  formed  :  the  lime-soda  series,  oligoclase, 
andesine,  and  labradorite ;  and  the  potash-soda  series,  as  yet 
imperfectly  known. 

The  felspars  crystallize  in  either  the  monoclinic  or  triclinic  sys- 
tems, but  the  forms  of  the  crystals  are  very  much  alike.  With 
few  exceptions,  these  minerals  are  of  pale  colours  and,  except 
when  decomposing,  are  very  hard. 

i.   Monoclinic  Felspars 

Orthoclase  is  a  potash  felspar  (K2O,  A12O3,  6  SiO2  =  K,  Al, 
Si3O8),  though  soda  may  replace  part  of  the  potash,  and  small 
quantities  of  lime  and  iron  are  usually  present.  Hardness  =  6, 
sp.  gr.  =  2.54-2.57.  Orthoclase  crystallizes  in  oblique  rhombic 
prisms  and  is  very  generally  twinned ;  there  are  two  sets  of  cleav- 
age planes,  which  intersect  at  a  right  angle  and  have  thus  given 
its  name  to  the  mineral.  Orthoclase  is  usually  dull  and  turbid, 
which  is  due  to  the  presence  of  various  alteration  products,  and 


FELSPATHOID   GROUP  I/ 

even  thin  sections  under  the  microscope  are  commonly  hazy. 
Sanidine  is  a  glassy,  transparent  variety  of  orthoclase,  which  is 
found  in  lavas  of  late  geological  date.  Its  clearness  is  due  to  the 
absence  of  the  decomposition  products,  which  render  ordinary 
orthoclase  turbid. 

2.    Triclinic  Felspars 

The  minerals  of  this  series  are  grouped  together  under  the  com- 
prehensive term  of  Plagioclase,  because  of  the  difficulty  of  distin- 
guishing them  from  each  other  under  the  microscope ;  they  are 
very  generally  characterized  by  polysynthetic  twinning,  which 
makes  fine  parallel  lines  on  the  basal  cleavage  planes.  Chemi- 
cally, they  are  soda,  lime,  or  lime-soda  felspars,  of  which  the 
latter  are  isomorphous  mixtures  of  albite  and  anorthite.  The  fol- 
lowing table  (from  Le"vy  and  Lacroix)  gives  the  composition  of 
the  various  members  of  this  series,  representing  the  soda-felspar 
constituent,  or  albite,  by  Ab,  and  the  lime-felspar  constituent,  or 
anorthite,  by  An  :  — 

NAME.  COMPOSITION.  SPECIFIC  GRAVITY. 

Albite Ab 2.62 

Oligoclase AbioAns 2.65 

Andesine Ab2Ani 2.67 

Labradorite     ....     AboAna 2.70 

Anorthite An 2.75 

It  will  be  observed  that  the  specific  gravity  increases  with  the 
lime  constituent,  and  the  fusibility  diminishes  in  the  same  propor- 
tion. Anorthite  is  decomposed  by  hydrochloric  acid,  labradorite 
is  slightly  attacked  by  it,  while  the  other  members  of  the  series 
are  not  affected. 

Anorthoclase  is  a  triclinic  potash-soda  felspar  (AbaO^),  but  is 
not  a  common  constituent  of  rocks. 


II.  THE  FELSPATHOID  GROUP 

These  minerals  are  very  closely  allied  to  the  felspars  in  chemi- 
cal composition,  but  differ  from  them  in  crystalline  form  and 
c 


1 8  THE  ROCK-FORMING   MINERALS 

physical  properties.  They  have  a  much  more  restricted  distribu- 
tion than  the  felspars,  but  have,  nevertheless,  an  important  bear- 
ing upon  the  classification  of  certain  groups  of  rocks  in  which  they 
occur. 

Nepheline  is  a  silicate  of  potash,  soda,  and  alumina  ((Na,  K)2O, 
A12O3,  2SiO2).  It  crystallizes  in  transparent  and  colourless  six- 
sided  prisms,  of  the  hexagonal  system.  H  =  5.5-6  ;  sp.  gr.  2.6. 
The  mineral  is  soluble  in  hydrochloric  acid,  gelatinous  hydrated 
silica  separating  out.  It  is  an  important  constituent  of  certain 
lavas. 

Leucite  is  composed  as  follows :  K2O,  A12O3,  4  SiO2,  with  some 
of  the  potash  replaced  by  soda.  It  crystallizes  in  twenty-four- 
sided  figures  (trapezohedrons),  which  belong  to  the  tetragonal 
system,  but  can  be  distinguished  from  the  isometric  only  by  very 
careful  measurement.  11  =  5.5-5.6;  sp.  gr.  =  2.44-2.56.  It  is 
slowly  attacked  by  hydrochloric  acid. 

Leucite  cannot  be  called  a  common  mineral,  but  its  significance 
will  be  better  seen  when  we  come  to  take  up  the  study  of  rocks. 

Analcite.  —  This  mineral  is  usually  regarded  as  a  decomposition 
product,  and  placed  among  the  zeolites  (see  below,  p.  21)  ;  but 
recent  investigations  make  it  very  probable  that  in  some  cases,  at 
least,  analcite  is  a  mineral  of  primary  origin.  Its  composition  is  : 
Na2O,  A12O3,  4  SiO2,  2  H2O.  Crystallizes  in  the  isometric  system, 
and  is  colourless  in  transmitted  light.  It  is  soluble  in  mineral 
acids,  with  separation  of  gelatinous  silica.  Sp.  gr.  =  2.15-2.28. 

III.  THE  MICA  GROUP 

These  minerals  have  a  complex  chemical  composition,  and  are 
so  variable  that  it  is  difficult  to  give  formulae  for  them ;  they  are 
silicates  of  alumina,  together  with  potash,  lithia,  magnesia,  iron,  or 
manganese.  There  is  a  difference  of  opinion  regarding  the 
crystalline  system  to  which  the  micas  should  be  referred.  When 
crystallized,  they  all  form  six-sided  prisms,  but  there  are  reasons 
for  believing  that  this  is  a  false  symmetry.  Usually  certain  micas 
are  referred  to  the  hexagonal,  and  others  to  the  orthorhombic 


AMPHIBOLE  AND   PYROXENE  GROUPS  19 

systems,  but  some  authorities  regard  them  all  as  monoclinic.  All 
varieties  have  a  remarkably  perfect  cleavage,  and  split  into  thin, 
elastic,  and  flexible  leaves,  by  which  they  may  be  readily  recog- 
nized. They  are  quite  soft,  and  most  of  them  may  be  scratched 
with  the  finger-nail. 

1.  Muscovite   may  be   selected   as   the   most   important   and 
wide-spread  of  the  numerous  alkaline  micas,  it  being  a  hydrated 
potash-mica,    with    the    general-  formula,    K2O,   3  A12O3,    6  SiO2, 
2  H2O.     It  is  a  lustrous,  silvery-white  mineral,  usually  transparent 
and  colourless  in  thin  leaves  ;  it  has  a  specific  gravity  of  2.76-3.1, 
and  a  hardness  of  2.1-3. 

Sericite  is  a  silvery  or  pale  green  form  of  muscovite,  which  is 
an  alteration  product  and  often  is  derived  from  a  felspar. 

2.  Lepidolite  is  a  mica  in  which  part  of  the  potash  has  been 
replaced  by  lithia. 

3.  Biotite  is  the  most  important  and  widely  disseminated  of  the 
numerous  dark-coloured,  ferromagnesian  micas.     This  mineral  is 
black  or  dark  green  in  mass,  and  smoky  even  in  thin  leaves ; 
chemically   it   is   a   hydrated  silicate   of  potash,   alumina,   iron, 
and  magnesia.     In  hardness  and  specific  gravity  it  differs  little 
from  muscovite. 

IV.  THE  A'MPHIBOLE  AND  PYROXENE  GROUPS 

These  two  groups  contain  parallel  series  of  minerals  of  similar 
chemical  composition,  but  differing  in  their  crystalline  form  and 
physical  properties.  In  composition  they  are  silicates  of  various 
protoxide  bases,  and  range  from  silicates  of  magnesia  to  those  of 
lime  and  lime-alumina,  while  silicate  of  iron  is  present  in  most 
of  them.  In  crystalline  form  they  belong  to  the  orthorhombic 
and  monoclinic  systems,  and  can  be  distinguished  by  their  cleav- 
age. The  pyroxenes  have  a  prismatic  cleavage  of  nearly  90°, 
while  in  the  amphiboles  the  angles  are  124°  30'  and  55°  30'. 
The  orthorhombic  amphiboles  are  rare  and  unimportant  as  rock- 
forming  minerals,  but  the  pyroxenes  of  this  form  are  widely  dis- 
tributed, though  less  so  than  the  monoclinic. 


2O  THE   ROCK-FORMING   MINERALS 

a.  Orthorhombic  Pyroxenes  are  silicates  of  magnesia  and  iron 
(Mg,  Fe)0,  Si02. 

1.  Enstatite  has  less  than  5%  of  FeO. 

2.  Bronzite  has  5-14%  of  FeO. 

3.  Hypersthene  has  more  than  14%  of  FeO. 

The  colour  becomes  darker  and  the  optical  properties  change 
with  the  increase  in  the  percentage  of  iron. 

b.  Monochnic  Pyroxenes. 

1.  Augite. — This  very  abundant  and  important  mineral  is  a 
silicate  of  lime,  magnesia,  iron,  and  alumina   (Ca,   Mg,  Fe)O, 
(Al,  Fe)2O3,  4  SiO2.     Sp.  gr.  =  3.3-3.5  ;  H  =  5-6.     It  crystallizes 
in  oblique  rhombic  prisms,  and  in  colour  is  green  to  black  and 
opaque. 

2.  Diallage  is  a  variety  of  augite,  usually  of  a  green  colour, 
which  is  distinguished  by  its  laminated  structure,  with  lustrous 
faces. 

c.  Mono  clinic  Amphiboles. 

1.  Hornblende,  like  augite,  which  it  closely  resembles  in  chemi- 
cal composition,  is  among  the  most  important   of  rock-forming 
minerals.     In  colour  it  is  usually  green,  brown,  or  black,  and  it 
crystallizes  in  modified  oblique  rhombic  prisms.     Sp.  gr.  =  2.9- 
3.5  ;  H  =  5-6. 

2.  Tremolite  is  a  silicate  of  magnesia  and  lime  (CaO,  MgO), 
SiO2.     This  mineral  is  pale  green  or  white  and  occurs  in  laminae  or 
long,  blade-like  crystals. 

3.  Actinolite  resembles  tremolite  in  composition,  with  the  addi- 
tion of  iron  (CaO,  MgO,  FeO)  SiO2.     Colour,  green  ;  sp.  gr.  = 
3-3.2;   usually  occurs   in  long  and  thin  crystals.     Asbestus  is  a 
fibrous  variety  of  tremolite  or  actinolite,  in  which  the  fibres  are 
often  like  flexible  threads  and  may  be  woven  into  cloth. 

V.   THE  OLIVINE  GROUP 

Olivine  is  the  only  mineral  of  this  group  of  sufficient  impor- 
tance to  require  mention ;  it  is  a  silicate  of  magnesia  and  iron 
2  (MgO,  FeO)SiO2,  though  the  percentage  of  iron  varies  greatly. 


TALC  AND  CHLORITE  GROUPS  21 

Sp.  gr.  =  3.2-3.5  ;  H  =  6.5-7.  Olivine  crystallizes  in  the  ortho- 
rhombic  system,  and  occurs  in  prisms,  flat  tables,  or  irregular 
grains.  Hydrochloric  acid  decomposes  the  mineral,  with  sepa- 
ration of  gelatinous  silica.  The  colour  varies  from  olive  green  to 
yellow,  or  it  may  be  colourless,  and  usually  the  irregular  grains 
look  like  fragments  of  bottle  glass. 

C.  OTHER  SILICATES,  CHIEFLY  DECOMPOSITION  PRODUCTS 

Many  of  the  complex  silicates,  when  long  exposed  to  the  action 
of  the  weather  and  of  percolating  waters,  become  more  or  less 
profoundly  changed.  One  of  the  commonest  of  these  changes  is 
hydration,  or  the  taking  up  of  water  into  chemical  union,  and  this 
may  be  accompanied  by  the  loss  of  soluble  ingredients,  or  the 
replacement  of  some  constituents  by  others. 

I.   ZEOLITES 

In  this  group  are  included  a  large  number  of  minerals,  which 
are  hydrated  silicates  of  alumina,  potash,  soda,  lime,  etc.  They 
all  contain  large  quantities  of  water  and  hence  boil  and  effervesce 
when  heated  before  the  blowpipe.  All  these  minerals  are  prod- 
ucts of  decomposition  and  do  not  occur  as  original  constituents 
of  rocks. 

II.  TALC  AND  CHLORITE  GROUPS 

i.  Chlorite.  — Under  this  name  are  grouped  a  number  of  closely 
allied  minerals,  which  are  hydrated  silicates  of  alumina,  magnesia, 
and  iron.  They  are  soft  minerals,  with  a  hardness  of  1-1.5  an^  a 
specific  gravity  of  2.6-2.96,  and  are  of  a  green  colour.  The  crystal- 
line form  is  somewhat  uncertain,  but  is  now  generally  regarded  as 
monoclinic,  with  a  pseudo-hexagonal  symmetry.  These  minerals 
are  laminated  and  split  readily  into  thin  leaves,  as  do  the  micas, 
from  which  they  may  be  distinguished  by  the  fact  that  the  leaves 
are  not  elastic. 

The  chlorites  result  from  the  decomposition  of  hornblende, 
augite,  or  the  magnesian  micas. 


22  THE   ROCK-FORMING   MINERALS 

2.  Talc  is  a  hydrated  silicate  of  magnesia,  3MgO,  4SiO2,  H2O  ; 
the  water  varies  in  amount  to  as  much  as  7%.     Sp.  gr.=  2.56-2.8  ; 
H  =  i.     It  is  of  a  white  or  pale  green  colour,  with  a  pearly  lustre 
and  a  greasy,  soapy  feeling  to  the  touch.     Talc  is  rarely  found 
crystallized  ;  the  crystals  have  a  false  hexagonal  symmetry,  and  it 
is  doubtful  whether  they  should  be  referred  to  the  orthorhombic 
or   monoclinic  systems.      Usually  it  occurs   in  flakes  or  foliated 
masses,  which  split  into  thin,  non-elastic  leaves.     Talc  results  from 
the  alteration  of  magnesian  minerals. 

3.  Steatite,  or  Soapstone,  has  the  same  composition  as  talc,  but 
is  not  foliated,  and  may  be  much  harder,  as  much  as  2.5. 

4.  Serpentine  is  a  hydrated  silicate  of  magnesia   and   iron : 
3  (MgO,  FeO)  2  SiO2,  2  H2O.    It  does  not  crystallize,  except  some- 
times in  pseudomorphs.    Sp.  gr.  =  2. 5-2. 65  ;  11  =  2.5-4.    Itsproper 
colour  is  green,  but  it  is  usually  mottled  with  red  or  yellow  by  iron 
stains.     Serpentine  is  generally  formed  from  the  decay  of  olivine, 
less  commonly  from  augite,  or  hornblende. 

Kaolinite  is  the  hydrated  silicate  of  alumina,  A12O3,  2  SiO2, 
2  H2O.  It  is  usually  soft  and  plastic,  but  orthorhombic  crystals  of 
pseudo-hexagonal  symmetry  may  be  sometimes  detected  with  the 
microscope.  Kaolinite  arises  from  the  decomposition  of  the  fel- 
spars and  especially  of  orthoclase. 

Glauconite  is  a  hydrated  silicate  of  alumina  and  iron,  with  small 
quantities  of  lime,  magnesia,  potash,  and  soda.  It  is  of  a  green 
colour,  soft  and  friable,  and  results  largely  from  the  decay  of 
augite. 

D.  CALCAREOUS   MINERALS 

i.  Calcite,  carbonate  of  lime,  CaCO3.  Sp.  gr.  =  2.72  ;  H  =  3. 
This  mineral  crystallizes  in  the  hexagonal  system,  in  a  great  vari- 
ety of  forms ;  rhombohedrons  and  scalenohedrons  are  common ; 
hexagonal  prisms  and  pyramids  less  so.  Cleavage  is  very  perfect, 
parallel  to  the  faces  of  a  rhombohedron,  and  the  mineral  breaks  up 
into  rhombohedrons  when  struck  a  sharp  blow.  Calcite  is  rapidly 
attacked,  even  by  cold  and  weak  acids,  CO2  escaping  with  effer- 


CALCAREOUS   MINERALS  23 

vescence.  When  pure,  as  in  Iceland  spar,  the  mineral  is  colour- 
less, very  transparent  and  lustrous,  and  displays  the  phenomenon 
of  double  refraction  strongly ;  hut  more  commonly  it  is  cloudy  or 
white,  or  stained  red  or  yellow  by  iron.  The  decomposition  of 
silicated  minerals  containing  lime  gives  rise  to  calcite,  and  as  this 
is  soluble  in  water  holding  CO2,  nearly  all  natural  waters  have 
more  or  less  of  it  in  solution.  It  is  widely  diffused  among  the 
rocks,  and  in  a  state  of  varying  purity  forms  great  masses  of 
limestone. 

2.  Aragonite  (CaCO3)  is  a  somewhat  harder  and  heavier  form 
of  calcite,  with  a  specific  gravity  of  2.93  and  a  hardness  of  3.5-4, 
and  crystallizes  in  compound  prismatic  forms  which  belo'ng  to  the 
orthorhombic  system.     It  has  not  the  marked  cleavage  of  calcite 
and  is  very  unstable  ;  when  heated  it  is  converted  into  calcite  and 
falls  into  tiny  rhombohedrons  of  that  mineral. 

3.  Dolomite  is  a  carbonate  of  lime  and  magnesia,  (Ca,  Mg)CO3, 
but  with  variable  proportions  of  the  two  bases ;  it  resembles  cal- 
cite in  appearance,  and  crystallizes  in  rhombohedrons  which  often 
have  curved  faces.     Sp.  gr.  =  2.8-2.9  ;  11=3.5-4.     Dolomite  may 
be  readily  distinguished  from  calcite  by  the  fact  that  cold  acids 
affect  it  but  little. 

4.  Selenite,  hydrated  sulphate  of  lime,  CaSO4,  2H2O.     Sp.  gr. 
=  2.31-2.33;    H  =1.5-2.      It   crystallizes   in   right   rhomboidal 
prisms,  belonging  to  the  monoclinic  system,  and  cleaves  into  thin 
non-elastic  leaves.     When  pure,  selenite  is  transparent  and  colour- 
less, but  is  often  stained  by  iron.     This  mineral  occurs  largely  in 
granular  masses,  called  gypsum,  from  which    plaster  of  Paris   is 
made  by  calcining  the  gypsum  and  so  driving  off  the  water  of 
crystallization.     Gypsum  is  slightly  soluble  and  is  present  in  most 
natural  waters.     Alabaster  is  a  gypsum  of  especially  fine  grain, 
mottled  in  pale  colours,  or  white. 

5.  Anhydrite,  CaSO4,  is  sulphate  of  lime  without  water ;  it  is 
harder  and  heavier  than  gypsum  (Sp.  gr.  =  2.9-2.98  ;  H  =  3-3.5), 
and  crystallizes  in  a  different  system,  the    orthorhombic.      The 
crystals  have  three  sets  of  cleavage  planes,  which  intersect  each 
other  at  right  angles. 


24  THE   ROCK-FORMING   MINERALS 

6.  Apatite  is  a  phosphate  of  lime,  with  chloride  and  fluoride  of 
calcium,  which  vary  in  relative  amounts,  3(Ca3P2O8),  2(Ca,  Cl,  F). 
Sp.  gr.  =  2.92-3.25  ;  H  =  5.     It  crystallizes  in  hexagonal  prisms, 
terminated  by  hexagonal  pyramids,  and  also  occurs  in  masses. 
The  original  and  unchanged  mineral  is  transparent  and  colourless, 
changing  on  alteration  to  opaque  brown  or  green.     Apatite  is  solu- 
ble in  acids,  and  in  water  containing  carbon  dioxide,  or  ammonia  ; 
it  is  thus  dissolved  out  of  the  rocks  and  widely  diffused  in  the  soils, 
where  it  forms  a  valuable  plant  food. 

7.  Fluor-spar,    fluoride    of  calcium,    CaF2.      Sp.  gr.  =  3.01- 
3.25  ;    H  =  4.      Crystallizes  in  the  isometric   system,  usually  in 
cubes,  and  has  a  perfect  octahedral  cleavage.     When  pure,  fluor- 
spar is  clear  and  colourless,  but  it  is  more  commonly  stained 
blue,  green,  yellow,  or  brown. 

E.    IRON  MINERALS 

1.  Haematite,  or  Specular  Iron,  is  ferric  oxide,  Fe2O3.    Sp.  gr.  = 
4.5-5.3;     H  =  6.5.       Crystallizes   in   rhombohedrons,   or   more 
commonly,  in  nodular  masses,  which  are  composed  internally  of 
very  flat  crystals.     The  colour  is  black  or  steel-grey,  which  be- 
comes red  when  the  mineral  is  finely  powdered.     Haematite  fre- 
quently contains  earthy  and   other  impurities  and  is  one  of  the 
most  important  ores  of  iron. 

2.  Limonite,   or   Brown    Haematite,  is   hydrated   ferric   oxide 
(2  Fe2O3,  3  H2O)  containing  more  than  14  %  of  water.     It  is  softer 
than  haematite  and  of  a  yellow  or  brown  colour.      Sp.  gr.  =3.6-4 ; 
H  =  5-5.5. 

3.  Magnetite  is  the  black  oxide  of  iron,  Fe3O4  (or  FeO,  Fe2O3). 
Sp.  gr.  =  4.9-5.2;    H  =  5.5~6.5.     Crystallizes   in    the    isometric 
system,   usually   in    octahedrons,    sometimes   in    dodecahedrons. 
This  mineral  is  strongly  magnetic  and  is  black  in  colour,  with  a 
bluish-black  metallic  lustre,  when  viewed  in  reflected  light.     Mag- 
netite is  widely  diffused  in  certain  classes  of  rocks,  and  also  occurs 
in  veins  and  beds,  which  form  an  important  source  of  supply  of 
the  metal. 


IRON   MINERALS  2$ 

4.  Ilmenite  is  a  titaniferous  ferric  oxide,  (Ti,  Fe)2O3.     Sp.  gr. 
—  4.5-5.2  ;  H  =  5-6.     When  crystallized,  this  mineral  is  rhom- 
bohedral,  but  is  generally  massive. 

5.  Siderite  is  ferrous  carbonate,  FeCO3.      Sp.  gr.  =  3.7-3.9; 
H  =  3.5-4.5.    Crystallizes  in  rhombohedrons,  the  faces  of  the  crys- 
tals frequently  much  curved,  and  often  the  crystals  are  very  much 
flattened.     When  fresh,  the  mineral  is  grey  or  brown.     It  is  but 
slightly  acted  on  by  cold  acids  ;  hot  acids  dissolve  it  with  efferves- 
cence.    Mixed  with  clay,  siderite  forms  clay  iron-stone,  a  valua- 
ble ore. 

6.  Iron  Pyrites,  bisulphide  of  iron,  FeS2.     Sp.  gr.  =  4.9-5.2; 
H  =  6-6.5.     Crystallizes  in  the  isometric  system,  usually  in  cubes, 
sometimes  in  dodecahedrons,  and  has  a  very  characteristic  brassy 
lustre  and  colour,  to  which  it  owes  the  popular  name  of  "  fools' 
gold."     It  is  very  hard,  cannot  be  scratched  with  a  knife,  and 
strikes  fire,  like  flint,  when  struck  with  steel.     The   mineral   is 
soluble  in  nitric  acid  :  it  is  widely  disseminated  in  the  rocks. 

7.  Marcasite,  or  White  Iron  Pyrites,  has  the  same  composition 
as  pyrites,  but  crystallizes  in  the  orthorhombic  system,  in  modified 
prisms,  but  more  commonly  occurs  in  nodular  masses,  with  a  radial 
structure.     It  has  the  same  hardness  as  pyrites,  but  is  not  quite 
so  heavy.    Sp.  gr.  =  4.68-4.85.     In  colour  it  is  paler  than  pyrites, 
with  a  tendency  to  grey,  green,  or  even  black.     It  decomposes 
very  readily  and  after  a  few  months'  exposure,  even  to  dry  air, 
often  crumbles  to  a  whitish  powder. 

The  iron  minerals  are  seldom  largely  represented  in  any  given 
rock,  with  the  exception  of  the  ore  beds,  but  iron  is  one  of  the 
most  widely  diffused  of  substances,  few  rocks  being  altogether  free 
from  it,  and  its  various  compounds  play  a  very  important  role  as 
colouring-matter  in  the  rocks.  Ferrous  carbonate  gives  no  colour 
to  the  rock  in  which  it  occurs,  and  such  rocks  are  apt  to  have  a  blue 
or  grey  tint,  due  to  other  substances,  both  organic  and  inorganic. 
When  such  rocks  are  exposed  to  the  action  of  air  and  moisture, 
ferric  oxide  and  ferric  hydrates  are  formed,  the  former  giving  a 
red  colour  and  the  latter  various  shades  of  yellow  and  brown. 


26  ROCK-FORMING   MINERALS 

A  blue  clay  containing  ferrous  carbonate  will,  when  fired  in  a 
kiln,  give  rise  to  red  bricks  or  pottery,  by  the  conversion  of  FeCO3 
into  Fe2O3.  Exposure  to  moist  air  produces  a  similar  effect  in 
nature,  and  the  contrast -in  colour  between  the  superficial  and 
deep-seated  layers  of  the  same  rock  is  often  as  great  as  between 
blue  clay  and  red  brick. 

Weathered  blocks  stained  red  on  the  outside  are  often  blue, 
grey,  or  nearly  black  on  the  inside,  the  change  not  having  pene- 
trated through  the  whole  mass.  Such  changes  are  most  con- 
spicuous in  the  sandstones,  because  their  porous  character  allows 
a  comparatively  free  circulation  of  air  and  water  through  them, 
but  similar  effects  are  frequently  to  be  observed  in  other  rocks 
also. 


PART    I 


DYNAMICAL    GEOLOGY 


WE  have  already  seen  that  the  chief  task  of  geology  is  to  con- 
struct a  history  of  the  earth,  to  determine  how  and  in  what  order 
the  rocks  were  formed,  through  what  changes  they  have  passed, 
and  how  they  reached  their  present  position.  The  logical  order 
of  treatment  might  seem  to  require  that  we  should  first  learn  what 
the  rocks  are,  of  what  they  are  composed,  and  how  they  are 
arranged,  before  attempting  to  explain  these  facts.  In  such  a 
study,  however,  we  should  meet  with  so  much  that  would  be  quite 
unintelligible,  that  a  more  convenient  way  will  be  to  begin  with 
a  study  of  the  agencies  which  are  at  work  upon  and  within  the 
earth,  and  which  tend  to  modify  it  in  one  or  other  particular.  In 
other  words,  we  must  employ  the  present  order  of  things  as  a  key 
by  means  of  which  to  decipher  the  hieroglyphics  of  the  past,  and 
proceed  from  what  may  be  directly  observed  to  past  changes 
which  can  only  be  inferred. 

We  might  assume  that  the  present  was  so  radically  different 
from  the  far-distant  past,  that  the  one  could  throw  no  light  upon 
the  other.  Such  an  assumption,  however,  would  be  most  illogical, 
for  there  is  nothing  to  support  it.  There  is  no  reason  to  imagine 
that  physical  and  chemical  laws  are  different  now  from  what  they 
have  always  been,  and  the  more  we  study  the  earth,  the  more 
clearly  we  perceive  that  its  history  is  a  continuous  whole,  deter- 
mined by  factors  of  the  same  sort  as  are  now  continuing  to 
modify  it.  Some  geologists  assume  that  these  agencies  have 

27 


28  DYNAMICAL  GEOLOGY 

always  acted  with  just  the  same  intensity  as  they  do  to-day ;  but 
this  assumption  is  neither  necessary,  nor  in  itself  probable. 
There  is,  on  the  contrary,  much  reason  to  believe  that  while  cer- 
tain forces  act  with  greater  efficiency  at  the  present  time  than 
they  did  in  the  past,  others  act  with  less. 

An  attentive  examination  of  the  changes  which  are  now  in  pro- 
gress on  the  surface  of  the  earth,  will  show  us  that  nothing  terres- 
trial is  quite  stable  or  unchangeable,  but  that  there  is  a  slow, 
ceaseless  circulation  of  matter  taking  place  on  the  surface  and 
within  the  crust  of  the  globe.  Matter,  chemistry  teaches  us,  is 
indestructible,  and,  disregarding  the  relatively  insignificant  amount 
of  material  which  reaches  us  from  outer  space  in  the  form  of 
meteorites,  the  sum  total  of  matter  composing  the  globe  remains 
constant.  But  while  practically  nothing  is  added  to  or  taken  away 
from  the  materials  which  make  up  the  earth's  crust,  ceaseless 
cycles  of  change  continually  alter  the  position,  physical  relations, 
and  chemical  composition  of  those  materials.  This  circulation  of 
matter  may  be  aptly  compared  to  the  changes  which  take  place 
in  the  body  of  a  living  animal,  only,  of  course,  they  are  of  a  differ- 
ent kind  and  are  effected  at  an  infinitely  slower  rate.  In  the  ani- 
mal body,  so  long  as  life  lasts,  old  tissues  break  down  into  simpler 
compounds  and  are  gotten  rid  of,  while  new  tissues  are  built  up 
out  of  fresh  material.  So,  on  the  earth  rock  masses  decay,  their 
particles  are  swept  away,  accumulate  in  a  new  place,  perhaps  far 
distant  from  their  source,  and  are  consolidated  into  new  rocks, 
which  in  their  turn  are  attacked  and  yield  materials  for  further 
combinations.  The  study  of  the  physical  and  chemical  changes 
in  the  bodies  of  animals  and  plants  constitutes  the  science  of 
physiology,  and  by  analogy  we  may  call  dynamical  geology  the 
physiology  of  the  earth's  crust.  Analogies,  however,  must  not 
be  pushed  too  far,  or  they  land  us  in  absurdities.  One  essential 
difference  between  the  earth  and  a  living  organism  suggests  itself 
at  once,  namely,  that  the  former  is  self-contained,  and  neither 
ejects  old  material  nor  receives  new,  but  employs  the  same  matter 
over  and  over  again  in  ever-varying  combinations.  The  animal  or 
plant,  on  the  contrary,  continually  takes  in  new  material  from 


DYNAMICAL  GEOLOGY  29 

without,  in  the  shape  of  food,  and  ejects  the  waste  resulting  from 
the  breaking  down  of  tissue. 

Although  the  earth  needs  no  fresh  supplies  of  matter,  its  dynami- 
cal operations  are,  to  a  very  large  extent,  maintained  by  energy  from 
without ;  namely,  from  the  sun.  The  circulation  of  the  winds  and 
waters,  the  changes  of  temperature,  and  the  activities  of  living 
beings,  all  depend  upon  the  sun's  energy,  and  were  that  with- 
drawn, only  such  changes  as  are  brought  about  by  the  earth's 
internal  heat  could  continue  in  operation. 

The  study  of  dynamical  agencies,  subterranean  and  surface, 
necessarily  gathers  together  an  enormous  mass  of  detail.  But  we 
need  concern  ourselves  with  only  so  much  of  this  as  throws  light 
upon  the  earth's  history,  so  that  the  sciences  of  dynamical  geology 
and  physical  geography,  though  having  much  in  common,  are 
essentially  distinct.  In  order  to  make  clear  the  operations  of  the 
forces  which  tend  to  modify  the  surface  of  the  earth,  it  is  neces- 
sary that  we  should  classify  and  arrange  them,  so  that  they  may 
be  treated  in  a  more  or  less  logical  order.  However,  in  making 
such  a  classification,  it  is  impossible  to  avoid  entirely  a  certain 
arbitrariness  of  arrangement,  since  we  must  consider  separately 
agencies  that  act  together.  Natural  phenomena  are  not  due 
to  single  causes,  but  to  combinations  and  series  of  causes,  and  yet, 
to  make  them  intelligible,  they  must  be  treated  singly  or  in  simple 
groups,  else  we  shall  be  confronted  by  a  chaotic  mass  of  uncorre- 
lated  details.  The  career  of  a  raindrop,  from  its  first  condensa- 
tion to  its  entrance  into  the  sea  through  the  mouth  of  some  river, 
is  a  continuous  one,  yet  rain  and  rivers  are  distinct  geological 
agencies  and  do  different  kinds  of  work.  Again,  the  very  im- 
portant way  in  which  the  various  dynamical  agents  modify,  check, 
or  augment  one  another,  must  not  be  overlooked  in  a  systematic 
arrangement  of  these  agents. 

Some  of  the  agencies  that  we  shall  consider  may  seem,  at  first 
sight,  to  be  very  trivial  in  their  effects,  but  it  must  be  remembered 
that  they  appear  so  only  because  of  the  short  time  during  which 
we  observe  them.  For  enormously  long  periods  of  time  they 
have  been  steadily  at  work,  and  their  cumulative  effects  must 


30  DYNAMICAL   GEOLOGY 

not  be  left  out  of  account  in  estimating  the  forces  which  have 
made  the  earth  what  we  find  it. 

Much  as  may  be  learned  by  the  study  of  the  operation  of  the 
forces  which  are  still  at  work  in  modifying  the  earth,  this  method 
of  study  is  yet  insufficient  to  solve  all  geological  problems.  Many 
of  the  changes  which  have  indisputably  taken  place  are  such  as 
no  man  has  ever  observed,  because  they  are  brought  about  so 
slowly  or  so  deep  down  within  the  crust  that  no  direct  observation 
is  possible,  and  we  can  only  infer  the  mode  of  procedure  by 
examining  the  result.  No  human  eye  has  ever  witnessed  the 
birth  of  a  mountain  range,  or  has  seen  the  beds  of  solid  rock 
folded  and  crumpled  like  so  many  sheets  of  paper,  or  observed 
the  processes  by  which  a  rock  is  changed  in  all  its  essential  charac- 
teristics j  "  metamorphosed,"  as  it  is  technically  called.  All  sudi 
problems  must  be  discussed  in  connection  with  structural  geology. 

The  dynamical  agencies  may,  primarily,  be  divided  into  two 
classes  :  I,  the  Subterranean  Agencies,  which  act,  or  at  least  origi- 
nate, at  considerable  depths  within  the  earth ;  and  II,  the  Surface 
or  Superficial  Agencies,  whose  action  takes  place  at  or  near  the 
surface  of  the  earth.  The  former  are  due  to  the  inherent  energy 
of  the  earth,  and  their  seat  is  primarily  subterranean,  though  their 
effects  are  very  frequently  apparent  at  the  surface.  These  agen- 
cies are  also  called  igneous  (from  ignis,  fire),  which  is  a  misnomer ; 
but  the  term  is  nevertheless  in  common  use. 

The  logical  order  of  treatment  of  these  subjects  is  to  begin  with 
the  subterranean  agencies,  because  the  most  ancient  rocks  of  the 
earth's  crust  were  doubtless  formed  by  these  forces,  and  the  circu- 
lation of  matter  upon  and  through  the  crust  started  originally 
from  igneous  rocks,  made  by  cooling  at  the  surface  of  a  molten 
globe. 


SECTION    I 

SUBTERRANEAN  OR   IGNEOUS  AGENCIES 

CHAPTER   II 
INTERIOR  CONSTITUTION  OF  THE  EARTH  —  VOLCANOES 

No  problems  of  geology  are  more  difficult  and  obscure  than 
those  connected  with  the  internal  constitution  of  the  earth,  and 
satisfactory  explanations  of  the  subterranean  agencies  have  not  yet 
been  devised.  This  is  because  the  interior  of  the  earth  is  so 
completely  beyond  the  range  of  direct  observation.  The  deepest 
boring  is  hardly  more  than  ^-^  part  of  the  earth's  radius,  and 
data  derived  from  the  temperature  observations  of  such  shallow 
openings  leave  a  great  deal  to  conjecture.  The  enormous  press- 
ures also  which  obtain  within  the  mass  of  the  globe,  are  such  that 
we  can  form  but  inadequate  conceptions  of  their  effects. 

Temperature  of  the  Earth's  Interior. — Volcanoes,  which  eject 
white-hot  and  molten  lavas,  and  thermal  springs,  which  pour  out 
floods  of  warm  or  even  boiling  water,  plainly  indicate  that  the 
interior  of  the  earth  is  highly  heated,  at  least  along  certain  lines. 
Direct  observations  tend  to  prove  that  this  high  temperature  is 
universally  diffused  through  the  mass  of  the  earth.  At  the  sur- 
face of  the  ground  and  for  a  short  distance  below  it,  the  tempera- 
ture varies,  like  that  of  the  air,  though  to  a  less  degree,  between 
day  and  night  and  between  different  hours  of  the  day.  Farther 
down, -the  daily  variation  ceases,  but  there  is  a  difference  of  tem- 
perature at  different  seasons,  it  being  lower  in  winter  than  in 
summer.  As  in  the  case  of  the  superficial  layer,  the  range  is  less 


32  INTERIOR  CONSTITUTION  OF  THE   EARTH 

extreme  than  in  the  air.  Penetrating  still  deeper,  we  come  to  A 
level  where  the  temperature  remains  the  same  throughout  the 
year,  and  is  also  the  same  as  the  annual  average  temperature  of 
the  air  at  the  same  locality  above  ground.  This  shows  that  the 
temperature  at  this  level  of  no  variation  is  determined  by  the  solar 
heat  and  other  climatic  factors.  The  depth  at  which  this  level  is 
situated  depends  upon  the  latitude  of  the  place  where  the  obser- 
vation is  made.  At  the  equator  it  is  only  three  or  four  feet  below 
the  surface  of  the  ground,  while  in  the  polar  regions  it  is  said  to 
be  several  hundred  feet  below.  This  does  not  imply  that  the 
effects  of  the  sun's  heat  penetrate  less  deeply  at  the  equator  than 
at  the  poles,  but  that  the  small  variations  of  atmospheric  tempera- 
tures in  the  tropics  are  equalized  at  shallow  depths  within  the 
earth.  In  temperate  regions  this  level  occupies  an  intermediate 
position,  and  at  the  latitude  of  New  York  is  found  at  a  depth  of 
about  fifty  feet. 

Beneath  the  level  of  no  variation  the  temperature  increases  with 
the  depth,  though  at  very  different  rates  in  different  localities.  In- 
creasing heat  with  increasing  depth  is  observed  in  all  deep  mines, 
tunnels,  and  borings,  and  in  some  cases  the  high  temperature  is  a 
very  effective  barrier  against  any  further  penetration  of  deep  shafts. 
In  locating  the  great  tunnels  under  the  Alps  the  engineers  have 
been  compelled  to  exercise  great  care,  lest  temperatures  exceed- 
ing those  in  which  men  can  work  should  be  encountered.  The 
average  rate  of  increase  of  temperature  is  usually  put  at  i°  Fahren- 
heit for  every  55  to  60  feet  of  descent,  but  recent  observations  made 
in  the  very  deep  shafts  of  the  Calumet  mine  in  the  Lake  Superior 
copper  region,  indicate  that  this  rate  is  perhaps  too  high.  Should 
this  rate  be  continued  regularly,  it  would  give  at  a  depth  of  25  to 
30  miles  a  heat  sufficient  to  melt  almost  any  rock,  at  atmospheric 
pressures. 

Physical  State  of  the  Earth's  Interior.  —  Astronomers  and 
geologists  agree  in  the  opinion  that  the  earth,  at  one  stage  in  its 
history,  was  a  nebulous  mass,  and  that  it  has  reached  its  present 
state  by  cooling  and  Contraction.  When,  however,  we  attempt 
to  determine  how  far  solidification  has  proceeded,  and  what  the 


PHYSICAL  STATE  OF  THE  INTERIOR  33 

present  condition  of  the  earth's  interior  is,  we  encounter  great 
difficulties.  On  this  subject  three  principal  hypotheses  have  been 
proposed.  ( i )  That  the  earth  is  practically  a  liquid  body,  covered 
only  by  a  relatively  thin  solid  crust.  (2)  That  it  is  substantially 
solid,  and  may  or  may  not  contain  localized  enclosures  of  molten 
matter  within  it.  (3)  That  it  has  a  very  large  solid  nucleus  and  a 
solid  crust,  and  interposed  between  the  two  a  layer  of  fused  mat- 
ter, upon  which  the  crust  floats  in  equilibrium. 

In  the  present  extremely  imperfect  state  of  knowledge  it  is  not 
possible  to  decide  definitely  between  these  conflicting  theories. 
The  first,  or  "thin  crust  theory,"  is  now  almost  entirely  abandoned, 
for  the  known  facts  do  not  require  us  to  believe  that  the  increase 
of  temperature  downward  keeps  on  indefinitely.  It  may  well  be 
the  case  that  the  globe  has  a  uniform  temperature  from  the  centre 
to  within  a  few  miles  of  the  surface.  If  this  be  true,  we  should 
observe  the  same  increasing  heat,  the  more  deeply  the  crust  is 
penetrated.  Again,  there  is  reason  to  believe  that  most  rocks  ex- 
pand on  melting,  and  thus  the  great  pressures  found  deep  within 
the  earth  would  necessitate  a  higher  temperature  to  melt  a  given 
rock  at  a  given  depth  than  at  the  surface.  We  do  not  know  how 
far  the  temperature  must  be  raised  to  overcome  the  increased  press- 
ure. More  important  are  the  astronomical  objections,  according 
to  which  the  behaviour  of  the  earth  is  like  that  of  a  rigid  and  not 
of  a  fluid  body. 

The  second  hypothesis,  that  the  earth  is  solid  to  the  centre,  is 
held  by  many  geologists  and  by  most  astronomers.  The  evidence 
in  its  favour  is  chiefly  the  fact  already  stated,  that  the  globe  be- 
haves, in  its  astronomical  relations,  like  a  rigid  solid.  This  view 
is  not  at  all  incompatible  with  the  belief  that  considerable  masses 
of  fused  material  may  be  contained  at  various  depths  within  the 
earth. 

The  third  hypothesis,  which  postulates  the  presence  of  a  fused 
layer  between  the  crust  and  the  nucleus,  is  held  by  many  geolo- 
gists and  astronomers.  According  to  this  view,  the  earth  first 
solidified  at  the  centre  from  pressure,  and  it  has  even  been  sug- 
gested that  the  molten  (as  distinguished  from  the  nebulous)  part 


34  VOLCANOES 

never  exceeded  a  superficial  layer  of  fifty  miles  in  thickness.  The 
next  step  was  the  formation  of  a  solid  crust  at  the  surface  by  cool- 
ing, leaving  beneath  it  a  layer  which  is  not  under  sufficient  pressure 
to  be  consolidated,  and,  being  protected  by  the  crust,  has  not  yet 
cooled  to  the  pokit  of  solidification.  Hence,  the  transition  from 
one  layer  to  another  is  gradual  and  not  abrupt.  Such  an  hypothe- 
sis is  believed  to  avoid,  on  the  one  hand,  the  astronomical  objec- 
tions to  a  substantially  fluid  earth,  and,  on  the  other,  certain 
geological  difficulties  in  the  way  of  accepting  the  belief  that  the 
earth  is  practically  solid  throughout.  But,  unfortunately,  we  are 
yet  far  too  ignorant  to  decide  between  these  different  views,  and 
it  is  merely  a  question  of  greater  or  less  probability,  according  to 
the  available  evidence. 

The  subterranean  or  igneous  agencies  may  be  conveniently 
classified  pnder  four  main  heads:  (i)  volcanoes,  (2)  thermal 
springs,  (3)  earthquakes,  (4)  changes  of  level  between  land  and 
sea.  Of  these,  the  second,  including  thermal  springs,  geysers, 
and  the  like,  will  be  considered  in  a  later  chapter,  because  they 
are  mixed  agencies,  and  cannot  be  well  understood  until  we  have 
learned  something  of  ordinary  springs  and  their  operations. 

I.   VOLCANOES 

A  volcano  is  usually  a  conical  hill  or  mountain,  having  an  open- 
ing, through  which  various  molten  or  solid  materials  are  cast  up. 
The  essential  part  of  the  volcano  is  the  opening  or  vent,  which 
establishes  a  connection  with  the  highly  heated  interior  of  the 
globe.  The  mountain,  when  present,  is  secondary,  and  is  formed 
by  the  materials  which  the  volcano  itself  has  piled  up ;  it  is  thus 
the  effect  and  in  no  sense  the  cause  of  the  phenomena. 

Present  Distribution  of  Volcanoes.  —  The  geographical  distribu- 
tion of  volcanic  vents  has  greatly  changed  at  different  periods  of 
the  earth's  history.  There  are  few  large  land  areas  which  do  not 
display  traces  of  former  volcanic  activity,  though  such  action  may 
have  died  out  ages  ago,  never  to  be  renewed,  and  no  active  vent 
be  found  for  great  distances  around  the  now  extinct  centres. 


DISTRIBUTION  35 

We  cannot  definitely  determine  the  number  of  vents  which  are 
at  present  in  activity  in  various  regions  of  the  earth,  because  a 
volcano  may  remain  dormant  for  centuries,  and  then  break  out 
again.  Almost  all  tradition  of  the  volcanic  nature  of  Vesuvius  had 
died  away  among  the  inhabitants  of  Italy,  until  the  dreadful  erup- 
tion of  the  year  79  A.D.  showed  that  it  had  only  been  slumbering. 
Many  volcanic  regions,  such  as  the  western  part  of  North  and 
South"  America,  and  the  East  Indian  islands,  have  been  known  to 
civilized  man  for  only  a  few  centuries,  and  in  such  regions  the  dis- 
tinction between  dormant  and  extinct  vents  cannot  always  be  made. 

Furthermore,  the  number  of  vents  is  constantly  changing,  new 
openings  forming,  and  old  ones  closing  up,  while  some  that  had 
escaped  observation  are  not  infrequently  discovered.  Another 
distinction  which  is  often  arbitrary,  is  that  between  independent 
volcanoes  and  mere  subsidiary  vents  connected  with  larger  ones. 
Several  submarine  volcanoes  have  been  observed,  but  it  is  alto- 
gether probable  that  many  more  exist  which  have  escaped  detec- 
tion. Making  these  allowances,  the  number  of  volcanoes  now 
active  may  be  estimated  at  about  328,  of  which  rather  more  than 
one-third  are  situated  in  the  continents,  and  the  remainder  on 
islands. 

The  active  volcanoes  are  not  scattered  hap-hazard  over  the  sur- 
face of  the  globe,  but  are  arranged  in  belts  or  lines,  which  bear  a 
definite  relation  to  the  great  topographical  features  of  the  earth. 
Two  of  these  belts  together  encircle  the  Pacific  Ocean ;  one  on 
'the  west  coast  of  the  Americas  runs  from  Alaska  to  Cape  Horn, 
the  other,  a  very  long  and  sinuous  band,  running  from  Kamchatka 
through  the  islands  parallel  to  the  east  coast  of  Asia,  the  East 
Indian  and  south  Pacific  islands,  to  the  Antarctic  circle,  where  it 
joins  the  American  band. 

A  third  band  occupies  a  ridge  in  the  eastern  bed  of  the  Atlantic, 
from  Iceland  to  St.  Helena,  from  which  arise  numerous  volcanic 
islands  and  submarine  vents.  South  of  Iceland  there  are  no 
known  volcanoes  for  a  great  distance,  until  the  Azores  are  reached, 
and  on  the  east  coast  of  the  Americas  are  none  at  all.  A  subsidi- 
ary volcanic  belt  passes  through  the  Mediterranean  to  Asia  Minor, 


36  VOLCANOES 

and  others  are  those  on  the  east  and  west  coasts  of  Africa  and  in 
the  Indian  Ocean.  The  prevalent  trend  of  the  belts  is  thus  in  a 
generally  north  and  south  direction. 

A  very  striking  fact  is  the  nearness  of  almost  all  active  volcanoes 
to  the  sea ;  by  far  the  greater  number  of  vents  are  upon  islands, 
and  those  of  the  continents  are,  with  few  exceptions,  not  far  from 
the  coasts.  Another  relation  which  should  be  noted,  is  that 
between  the  volcanic  bands  and  the  mountain  chains,  the  bands 
running  parallel  to  or  coinciding  with  the  mountains,  as  in  the 
great  volcanoes  of  the  Andes.  Not  all  coast  lines  or  all  moun- 
tain chains  have  volcanoes  associated  with  them,  but  where  the 
mountains  are  near  the  seashore,  volcanoes  are  usually,  though 
not  invariably,  found.  The  seat  of  volcanic  activity  is  frequently 
shifted,  as  we  have  learned,  and  it  has  been  observed  that  this 
activity  tends  to  die  out  of  the  older  rocks  and  to  make  its  appear- 
ance in  those  of  a  later  date. 

Volcanic  Eruptions.  —  The  phenomena  displayed  by  different 
volcanoes,  or  even  by  the  same  volcano  at  different  times,  vary 
greatly.  It  often  seems  difficult  to  believe  that  similar  forces  are 
involved,  and  that  the  divergences  are  due  merely  to  different  cir- 
cumstances attending  the  outbreak.  A  careful  comparison,  how- 
ever, of  the  varying  phenomena  brings  to  light  a  fundamental  like- 
ness in  them  all.  Some  vents,  like  Stromboli  in  the  Mediterranean, 
are  in  an  almost  continual  state  of  eruption  of  a  quiet  kind ;  others, 
like  Vesuvius,  have  long  periods  of  dormancy,  broken  by  eruptions 
of  terrible  violence.  In  a  general  way,  it  may  be  said  that  the 
longer  the  period  of  quiet,  the  more  violent  and  long-continued 
will  the  subsequent  eruption  be,  while  weak  eruptions  and  those 
of  short  duration  recur  at  brief  intervals. 

The  comparatively  gentle  operations  of  Stromboli  give  the  ob- 
server an  opportunity  to  learn  what  are  the  essential  phenomena 
of  a  volcanic  eruption.  Though  occasionally  breaking  out  with 
violence,  Stromboli  is  for  long  periods  in  such  exact  equilibrium 
that  barometric  changes  have  a  marked  effect  upon  its  activity, 
and  the  Mediterranean  sailors  make  use  of  it  as  a  weather-signal. 
The  floor  of  the  crater  is  formed  by  hardened  lava,  the  cracks  in 


PHENOMENA   OF   ERUPTION  37 

which  glow  at  night,  with  the  heat  of  the  molten  mass  below,  and 
which  is  perforated  by  various  openings.  From  some  of  these 
steam  is  given  out,  from  others  molten  lava  wells  up  occasionally. 
In  openings  of  a  third  class  the  lava  may  be  seen  rising  and  sink- 
ing, until  a  great  bubble  forms  on  its  surface  and  bursts  with  a 
loud  roar,  scattering  the  hardened  lava  scum  about  the  crater,  in 
fragments  of  various  sizes,  some  very  fine,  others  coarse.  The 
bubble  proves  to  be  of  steam,  and  when  set  free,  the  steam  globule 
rises  to  join  the  cloud  which  always  overhangs  the  mountain. 
The  bursting  of  the  steam  bubble  is  followed  by  a  rush  of  steam 
through  the  mass  of  the  lava,  the  pressure  is  relieved,  and  the  lava 
column  sinks  down  out  of  sight,  until  the  steam  pressure  again 
accumulates  and  the  performance  is  repeated. 

Evidently,  one  active  agent  in  these  phenomena  is  imprisoned 
steam  in  its  struggles  to  escape.  Different  as  are  the  manifesta- 
tions at  other  volcanoes,  steam  is  an  important  cause  of  the  erup- 
tion in  all  cases,  though  the  conditions  under  which  it  acts  vary 
widely.  Little  or  no  combustion  is  involved,  and  that  not  as  a 
cause,  but  as  an  effect  of  the  activity. 

In  Vesuvius  essentially  the  same  phenomena  may  be  observed, 
but  on  a  far  grander  and  more  terrible  scale.  Earthquakes  usually 
announce  the  coming  eruption,  increasing  in  force  until  the  out- 
break occurs.  Terrific  explosions  blow  out  fragments  of  all  sizes, 
from  great  blocks  to  the  finest  and  most  impalpable  dust.  The 
finer  fragments  arise  chiefly  from  the  scattering  of  the  partly  hard- 
ened lava  by  the  force  of  the  explosion,  but  in  part  also  from  the 
crashing  together  of  the  blocks  as  they  rise  and  fall  through  the 
air.  Inconceivable  quantities  of  steam  are  given  off  with  a  loud 
roar,  which  is  awe-inspiring  in  its  great  and  steady  volume.  The 
condensation  of  such  masses  of  vapour  produces  torrents  of 
rain,  which,  mingling  with  the  "  ashes  "  and  dust,  gives  rise  to 
streams  of  hot  mud  that  flow  for  long  distances  and  are  often 
exceedingly  destructive.  Great  floods  of  molten  rock,  or  lava, 
issue  from  the  crater,  or  burst  their  way  through  the  walls  of  the 
cone,  and  pour  down  the  mountain  side,  until  they  gradually 
stiffen  by  cooling. 


38  VOLCANOES 

The  first  recorded  eruption  of  Vesuvius  is  that  which  occurred 
in  79  A.D.  and  is  described  in  two  letters  written  to  Tacitus  by  the 
younger  Pliny.  In  this  frightful  paroxysm  little  or  no  molten  lava 
was  ejected,  but  the  old  cone  was  partly  blown  away  (its  remnant 
now  forms  the  outer  ring,  called  Monte  Somma),  and  so  enormous 
was  the  quantity  of  ashes  that  at  Misenum,  across  the  bay  of 
Naples,  the  sun  was  darkened,  as  Pliny  reports,  "  not  as  on  a 
moonless  cloudy  night,  but  as  when  the  light  is  extinguished  in  a 
closed  room  ...  In  order  not  to  be  covered  by  the  falling  ashes 
and  crushed  by  their  weight,  it  was  often  necessary  to  rise  and 
shake  them  off."  Herculaneum  was  overwhelmed  with  floods  of 
ashes  mixed  with  water,  while  Pompeii  was  completely  buried  in 
dry  ashes  and  small  fragments. 

This  first  historical  eruption  of  Vesuvius  was  thus  of  the  type 
known  as  explosive,  which  is  exhibited  in  its  extreme  form  by 
several  of  the  East  Indian  volcanoes,  and  preeminently  by  Kra- 
katoa,  the  eruption  of  which  in  1883  was  the  most  frightful  ever 


FIG.  6. —  Profiles  of  Krakatoa.    The  full  line  is  the  present  condition,  the  dotted 
line  the  condition  before  the  eruption  of  1883.     (Judd.) 

recorded.  This  volcanic  island,  situated  in  the  Strait  of  Sunda, 
was  little  known,  except  that  it  had  been  in  eruption  in  1680.  As 
the  island  was  uninhabited,  the  earliest  stages  of  the  outburst  were 
not  observed,  but  on  May  20  a  great  cloud  of  steam  was  seen  over 
the  vent.  The  catastrophe  occurred  in  August,  when,  besides  the 
fearful  devastation  caused  by  the  disturbances  of  the  sea  on  the 
coasts  of  Sumatra  and  Java,  the  island  itself  was  almost  annihilated. 
Hardly  one-third  of  its  original  surface  w as  left  above  water,  and 
where  formerly  was  land  are  now  depths  of  100  to  150  fathoms  of 
water.  The  force  of  the  explosion  produced  waves  in  the  atmos- 
phere which  were  propagated  around  the  whole  earth,  and  the 
first  one  was  observed  in  Berlin  ten  hours  after  the  explosion. 


PHENOMENA   OF   ERUPTION 


39 


The  ejected  materials  were  all  fragmentary  and  of  an  incredible 
volume ;  ashes  were  distributed  over  an  area  of  300,000  square 
miles,  the  greater  part  falling  within  a  radius  of  eight  miles  around 
the  island ;.  stretches  of  water  that  had  had  an  average  depth  of 
117  feet  were  so  filled  up  as  to  be  no  longer  navigable.  Enor- 
mous masses  of  pumice  floated  upon  the  sea  and  stopped  naviga- 
tion except  for  the  most  powerful  steamers. 

These  tremendous  explosions,  even  when  they  do  not  tear  out 
one  whole  side  of  the  mountain,  as  in  the  case  of  Krakatoa  and 


FIG.  7.  — Crater  Lake,  Oregon.     This  crater  ring  is  believed  to  have  been  formed 
by  a  draining  away  of  the  lava  from  below,  and  not  by  explosion.      (U.  S.  G.  S.) 

Monte  Somma,  may  blow  off  the  top  and  thus  leave  a  great 
crater  ring  many  miles  in  circumference,  within  which  subse- 
quent eruptions  may  build  up  a  new  cone.  When  the  volcanic 
activity  dies  out,  the  ring  may  be  filled  with  water,  forming  a 
circular  lake. 

Just  the  opposite  extreme  from  these  explosive  eruptions  is  to 
be  found  in  the  volcanoes  of  the  Sandwich  Islands,  such  as  Mauna 


4°  "VOLCANOES 

Loa  and  Kilauea.  Here  the  eruptions  are  usually  not  heralded  by 
earthquakes  ;  the  lava  is  remarkably  fluid  and  simply  wells  up  over 
the  sides  of  the  crater,  pouring  down  the  sides  of  the  mountain  in 
streams  which  flow  for  many  miles.  More  commonly  the  walls  of 
the  crater  are  unable  to  withstand  the  enormous  pressure  of  the 
lava  column,  and  the  molten  mass  breaks  through  at  some  level 
below  the  crater,  rising  through  the  fissure  in  giant  fountains, 


FIG.  8.  — Crater-floor  of  Kilauea,  showing  the  lava  lake,  Hale-mau-mau.     (Photo- 
graph by  Libbey.) 

sometimes  1000  feet  high.  Even  in  the  ordinary  activity  of 
Kilauea  jets  of  30  and  40  feet  in  height  are  thrown  up.  Hardly 
any  ashes  or  other  fragmental  products  are  formed,  but  the  clouds 
of  steam,  the  invariable  accompaniments  of  volcanic  outbursts,  are 
present. 

Between  such  extremes  as  the  Hawaiian  volcanoes,  on  the  one 
hand,  and  the  explosive  East  Indian  type  (Krakatoa),  on  the 
other,  we  may  find  every  intermediate  gradation.  Everywhere 
imprisoned  steam  appears  to  be  an  important  agent,  while  the 


VOLCANIC   PRODUCTS  41 

differences  are  due  to  the  varying  degrees  of  accumulated  pressure,*' 
the  resistance  to  be   overcome,   the  character  of  the   lava,   the 
manner  in  which  the  steam  is  generated,  and  similar  factors. 

Volcanic  Products. — These  form  the  most  important  part  of  the 
subject  from  the  geological  point  of  view,  because  they  contribute 
largely  to  the  permanent  materials  of  the  earth's  crust.  We  meet 


FIG.  9.  — Another  view  of  the  crater-floor  and  walls  of  Kilauea.      (Photograph 

by  Libbey.) 

with  such  materials  of  all  geological  ages,  sometimes  developed  on 
a  vast  scale.  The  study  of  volcanic  products  is  the  key  which 
enables  us  to  comprehend  the  great  group  of  rocks  which  are 
called  igneous,  though,  as  we  shall  see  later,  by  no  means  all  of 
these  were  poured  out  on  the  surface  of  the  ground. 

Volcanic  products  are  of  three  kinds:  (i)  lava,  or  molten 
rock;  (2)  fragmental  material,  including  blocks,  lapilli,  bombs, 
the  so-called  volcanic  ashes,  cinders,  and  the  like ;  (3)  gases  and 
vapours. 


42  VOLCANOES 

(i)  Lava. — A  lava  is  a  more  or  less  completely  melted  rock; 
the  degree  of  fluidity  varies  greatly  in  different  lavas,  but  is  rarely, 
if  ever,  perfect.  Instead  of  being  a  true  liquid,  a  lava  ordinarily 
consists  of  larger  and  smaller  crystals,  embedded  in  a  pasty  mass, 
which  is  saturated  with  steam  and  gases.  The  degree  of  fluidity 
depends  upon  several  factors,  the  most  obvious  of  which  is  tempera- 


FlG.  10.  —  Edge  of  Hale-mau-mau,  showing  the   ropy  forms   of  the   highly  fluid 
lava,  when  cooling.     (Photograph  by  Libbey.) 

ture ;  the  more  highly  heated  the  mass  is,  the  more  perfectly  will 
it  be  melted.  The  quantity  of  imprisoned  gases  and  vapours 
present  has  also  an  important  effect,  and  some  lavas  appear  to  owe 
nearly  all  their  mobility  to  these  vapours.  A  third  and  most  sig- 
nificant factor  is  the  chemical  composition.  Those  lavas  which 
contain  high  percentages  of  silica  (SiO2),  the  acid  lavas,  are  much 
less  readily  fusible  than  the  basic  lavas,  in  which  the  percentage  of 
silica  is  lower.  The  difference  in  the  proportion  of  silica  present 


LAVA 


43 


is  associated  with  other  chemical  differences  which  have  a  similar 
effect  upon  fusibility,  the  basic  kinds  having  much  more  lime, 
magnesia,  and  iron  in  them,  which  act  as  fluxes. 

The  experiments  of  Barns  on  the  fusibility  of  lavas,  which  he 
divides  into  three  groups,  resulted  as  follows:  (i)  Certain  lavas 
fuse  readily  (2250°  F.) ;  these  are  of  basic  composition  and  are 


FlG.  ii. —  Lava  flow  on  Vesuvius,  showing  slaggy  and  scoriaceous  surface. 

made  up  of  lime-soda  felspars  (see  p.  I6),1  the  augitic  and  allied 
•  ferro-magnesian  minerals  (p.  20),  and  iron  oxide,  but  rarely  have 
quartz  (p.  15).  (2)  A  second  group  is  of  medium  fusibility  (2520° 
F.),  and  is  made  up  of  lime-soda  felspars,  augitic  or  hornblende 
minerals,  and  frequently  quartz.  (3)  The  third  series  melt  with 
difficulty  (2700°  F.),  and  remain  pasty  at  even  3100°  F.  These 

1  For  the  sake  of  convenience,  the  minerals  are  all  considered  together  in  Chap- 
ter I,  but  the  student  will  greatly  facilitate  his  acquaintance  with  them  by  referring 
to  the  description  of  every  mineral  that  he  finds  mentioned  in  the  text,  and  es- 
pecially by  examining  specimens. 


44  VOLCANOES 

are  acid  lavas,  and  are  composed  of  potash  felspars  (p.  16),  with 
quartz,  hornblende,  or  mica.  Lavas  which,  like  those  of  the  Sand- 
wich Islands,  are  notably  fluid,  are  always  of  basic  composition. 

When  a  lava  stream  reaches  the  surface  of  the  ground,  the 
imprisoned  vapours  immediately  begin  to  escape  and  the  surface 
of  the  molten  mass  to  cool  and  harden.  The  surface  layers  are 
blown  by  the  steam  bubbles  into  a  light,  frothy  or  slaggy  consist- 


FiG.  12.— Lava-tunnel,  and  "Spatter-cone"  formed  by  escaping  steam,  Kilauea. 
(Photograph  by  Libbey.) 

ency,  forming  "  scoriae  "  or  cindery  masses.  The  motion  of  the 
lava  breaks  up  this  thin  crust  into  loose  slabs  and  blocks,  and  on 
the  advancing  front  of  the  stream  these  loose  masses  rattle  down 
over  one  another  in  the  wildest  confusion.  The  less  perfectly 
fused  lavas  are  soon  covered  with  heaped-up  cindery  blocks,  while 
the  more  completely  fluid  lavas  are  characterized  by  curiously 
twisted,  ropy  surfaces,  such  as  may  be  observed  in  the  slag  from  an 
iron  furnace. 


LAVA   STREAMS 


45 


The  front  of  a  lava  stream  advances,  not  by  gliding  over  the 
ground,  but  by  rolling,  the  bottom  being  retarded  by  the  friction 
of  the  ground  and  the  top  moving  faster,  so  that  it  is  continually 
rolling  down  at  the  curved  front  end  and  forming  the  bottom. 
Thus,  the  scoriae,  though  formed  mostly  on  the  top  of  the  stream, 
are  rolled  beneath  it,  and  the  whole  is  enclosed  in  a  cindery 


FIG.    13.  —  Lava  stalactites   and   stalagmites,   in   lava-tunnel,   Kilauea. 
graph  by  Libbey.) 


(Photo- 


envelope.  Or  the  flow  may  be  checked  by  the  mass  of  cinders, 
until  the  fluid  lava  bursts  through  them  in  a  fresh  stream.  The 
scoriaceous  mass  is  a  non-conductor  of  heat,  and  greatly  retards 
the  cooling  of  the  interior  mass,  which  may  remain  hot  for  many 
years.  The  arched  surface  of  cindery  blocks  may  become  self- 
supporting,  and  then  the  still  fluid  mass  will  flow  away  from 
beneath  it,  leaving  long  tunnels  or  caverns.  These  tunnels  are 

especially  well  shown  in  Iceland  and  the  jkmdyyich  Islands. 

*~f 


46  VOLCANOES 

The  distance  to  which  lava  streams  extend  and  the  rapidity 
with  which  they  move  are  determined  by  the  abundance  and 
fluidity  of  the  lava  and  the  slope  over  which  it  flows.  Some  lavas 
are  so  liquid  that  they  flow  for  many  miles,  even  down  moderate 
slopes,  while  others  are  so  pasty  that  they  stiffen  and  set  within  a 
short  distance  of  the  vent,  even  on  steep  grades.  Ordinarily  the 
motion  soon  becomes  very  slow,  though  thoroughly  melted  masses 
pouring  down  steep  slopes  may,  for  a  short  time,  move  very  swiftly. 
One  of  the  lava  floods  from  Mauna  Loa  moved  fifteen  miles  in 
two  hours,  and  for  shorter  distances  much  higher  rates  of  speed 
have  been  observed ;  but  this  is  very  exceptional. 

The  cooling  of  the  surfaces  of  the  lava  stream  takes  place  rap- 
idly, while  the  interior  cools  but  slowly,  and  great  thicknesses 
require  very  long  periods  of  time  to  become  entirely  cold.  The 
differences  in  the  rate  of  cooling  produce  very  strongly  marked 
varieties  in  the  appearance  and  texture  of  the  resulting  rock.  The 
portions  which  have  chilled  and  solidified  very  quickly  are  glassy 
and  form  the  volcanic  glass,  obsidian.  If  the  swiftly  cooling  por- 
tions have  been  much  disturbed  by  the  bubbles  of  steam  and 
vapours,  they  are  made  light  and  frothy ;  in  some  cases,  as  in 
pumice,  they  will  float  upon  water.  Otherwise,  the  glass  is  solid 
and  is  usually  very  dark  in  colour,  resembling  an  inferior  bottle 
glass  in  appearance.  Microscopic  examination  shows  minute, 
hair-like  bodies  in  the  glass,  which  are  called  crystallites,  and  rep- 
resent the  incipient  stages  of  crystallization. 

Passing  inward  from  the  surface  of  the  lava  stream,  we  find 
the  steam  bubbles  becoming  rarer,  until  they  cease  altogether,  the 
vapours  having  escaped  while  the  lava  was  still  so  soft  that  the 
bubble  holes  soon  collapsed.  At  the  same  time  the  glassy  texture 
of  the  rock  is  replaced  by  a  stony  character,  which  the  microscope 
shows  to  be  due  to  the  formation  of  crystals  too  minute  to  be  rec- 
ognized by  the  unaided  eye.  Still  deeper  in  the  rock  the  stony 
texture  passes  gradually  into  an  obviously  crystalline  one  ;  and  the 
slower  the  cooling,  the  larger  will  these  crystals  be,  though  in  lava 
streams  which  have  cooled  on  the  surface  of  the  ground,  the  whole 
mass,  even  of  the  deeper  parts,  is  never  coarsely  crystalline. 


LAVA  47 

Large  crystals  are,  it  is  true,  very  often  found  in  lavas,  but  these 
were  formed  before  the  ejection  of  the  mass  from  the  volcano. 
Such  crystals  frequently  contain  enclosures  of  glass,  which  indi- 
cate that  the  crystallization  went  on  while  the  surrounding  mass 
was  still  fluid.  The  edges  and  angles  of  these  crystals  are  often 
corroded  by  the  action  of  the  melted  portion  of  the  lava,  and  the 
motion  of  the  stream  often  cracks  them.  These  facts  go  to  prove 
that  the  large  crystals  were  complete  when  the  lava,  as  a  whole, 
was  still  fluid  and  in  motion.  Stromboli  ejects  great  numbers  of 
perfect  crystals  of  augite,  which  must  have  existed  in  the  molten 
lava  of  the  vent.  The  lavas  which  contain  large  crystals  embedded 
in  a  fine  stony  or  glassy  base  are  said  to  be  of  a  porphyritic 
texture. 

It  is  important  to  remember  that  all  these  various  textures  may 
be  found  in  one  continuous  rock  mass,  and  bear  witness  as  to  the 
circumstances  under  which  each  part  cooled  and  solidified.  These 
textures  also  recur  again  and  again  in  ancient  rocks  and  enable  us 
to  determine  their  volcanic  origin.  The  processes  of  rock  destruc- 
tion have  in  many  cases  laid  bare  deep-seated  masses  which  were 
plainly  once  melted  like  true  lavas,  but  which  have  cooled  very 
slowly  and  under  great  pressures.  In  such  rocks  the  texture  is 
usually  coarsely  crystalline  and  shows  no  traces  of  glass  or  scoriae. 
Between  the  surface  lava  flows  and  such  deep-seated  reservoirs 
every  form  of  transition  may  be  traced,  often  in  continuous  rock 
masses. 

Where  several  successive  lava  flows  issue  from  one  vent,  at  in- 
tervals which  allow  one  stream  to  be  consolidated  before  the  next 
is  poured  out  over  it,  a  rough  bedding  or  stratification  results, 
each  flow  being  perfectly  distinguishable  when  seen  in  section. 
Deceptive  resemblances  to  the  true  stratification  of  sedimentary 
rocks  (see  p.  145)  may  thus  arise,  especially  when  the  exposed 
section  is  short.  But  the  wedge-like  form  of  the  sheets,  the  absence 
of  bedding  within  the  limits  of  each  flow,  and  the  nature  of  the 
rock  itself,  always  enable  us  to  distinguish  these  masses  from  the 
sediments  which  have  been  stratified  by  the  sorting  power  of 
water. 


48 


VOLCANOES 


A  mass  of  lava,  when  it  cools  and  solidifies,  necessarily  contracts, 
and  since  the  cohesion  of  the  mass  is  insufficient  to  allow  it  to 
contract  as  a  whole,  it  must  crack  into  blocks,  separated  by 
fine  crevices,  which  are  called  joints.  The  mutual  relations  of 
the  jointing  planes,  and  the  consequent  shape  of  the  blocks,  are 
determined  largely  by  the  grain  of  the  lava  and  its  degree  of 


FlG.   14.  —  Stream  gorge,   island  of  Hawaii;   displaying   modern  columnar   lava. 
(Photograph  by  Libbey.) 

homogeneousness.  In  fine-grained  (and  some  coarse-grained) 
homogeneous  lavas  the  jointing  is  apt  to  be  very  regular,  and  to 
give  rise  to  prismatic  or  columnar  blocks,  which  are  usually  hex- 
agonal. This  shape  is  due  to  the  fact  that  the  formation  of  hex- 
agons requires  less  expenditure  of  work  than  other  figures,  and  is 
produced  by  the  intersection  of  systems  of  three  cracks,  radiating 
from  equidistant  points  at  angles  of  120°.  The  long  axes  of  the 
prisms  are  at  right  angles  to  the  cooling  surface.  Starch  and 
fire-clay,  which  shrink  on  drying,  joint  in  the  same  way.  The 


JOINTING   OF  LAVA 


49 


coarser  and  more  heterogeneous  lavas  usually  break  up  into  blocks 
of  irregular  size  and  shape. 


FlG.  15.  —  Obsidian  Cliff,  Yellowstone  Park.     Hexagonal  jointing.      (U.  S.  G.  S.) 
E 


50  VOLCANOES 

It  must  not  be  inferred  that  the  joints  of  all  rocks  are  due  to 
shrinkage  on  cooling.  It  will  be  shown  in  a  subsequent  chapter 
that  such  is  very  far  from  being  the  case. 

Not  all  the  lava  produced  in  and  around  a  volcanic  vent  can 
reach  the  surface.  Some  of  it  may  be  forced  horizontally  be- 
tween the  beds  of  the  surrounding  rocks,  thus  forming  intrusive 
sheets,  which,  when  exposed  in  section,  may  be  readily  distin- 
guished from  surface  flows  by  the  fact  that  they  have  consolidated 
under  pressure,  and  hence  have  no  slag  or  scoriae  associated 
with  them.  Other  portions  of  the  lava  will  fill  up  vertical  fissures 
in  the  volcanic  cone  or  in  the  underlying  rocks,  and,  solidifying 
in  these  fissures,  form  dikes.  Such  a  fissure,  twelve  miles  in 
length  and  filled  with  molten  lava,  was  observed  by  Sir  Charles 
Lyell  in  the  neighbourhood  of  ^Etna.  In  the  great  eruption  of 
Skaptar  Jokul  (Iceland)  in  1783  lava  was  poured  out  at  several 
points  along  a  line  two  hundred  miles  long,  and  doubtless  this 
was  a  great  lava-filled  fissure  which  consolidated  into  a  dike. 

We  thus  see  that  the  molten  masses  may  not  all  well  up 
through  the  crater  of  a  volcano,  but  will  seek  egress  along  the 
line  of  least  resistance,  wherever  that  happens  to  be,  breaching 
the  walls  of  the  volcanic  cone,  rising  up  through  vertical  fissures, 
or  forcing  their  way  as  intrusive  sheets  between  the  beds  of  pre- 
existing rocks.  In  these  various  situations  the  different  rates  of 
cooling  produce  many  varieties  of  rocks,  though  the  original 
molten  mass  may  have  been  nearly  or  quite  identical  in  all  of 
them. 

Lavas  \yhich  flow  into  the  sea  from  a  terrestrial  vent,  or  are 
poured  out  from  a  submarine  one  show,  as  a  rule,  but  little 
difference  from  those  which  solidified  on  land,  because  the  rapid 
formation  of  a  cindery  crust  will  protect  the  hot  lava  from  contact 
with  the  water.  Sometimes,  however,  the  sudden  chill  will  cause 
the  lava  to  disintegrate  into  a  mass  like  black  sand. 

(2)  Fragmental Products. — This  divisioiPmcludes  all  the  mate- 
rials which  are  ejected  from  the  volcano  in  a  solid  state.  These 
are  of  all  sizes  and  shapes,  from  huge  blocks  weighing  many  tons, 
down  to  the  most  impalpable  dust,  which  the  wind  will  carry  for 


FRAGMENTAL   PRODUCTS  51 

thousands  of  miles.  The  very  large  blocks  are  commonly  frag- 
ments of  the  older  rocks,  through  which  the  volcanic  vent  has 
burst  its  way,  tearing  a  great  hole  and  scattering  the  fragments 
widely.  For  fifteen  miles  around  the  lofty  volcano  of  Cotopaxi 
in  Ecuador  lie  great  blocks  of  this  nature,  some  of  them  measur- 
ing nine  feet  in  diameter. 

More  important  and  much  more  extensively  formed  and  widely 
spread  are  those  fragmental  products  which  are  derived  from  the 
lava  itself.  The  more  violently  explosive  the  eruption,  the  greater 
the  proportion  of  the  lava  that  will  be  blown  into  fragments.  In 
such  eruptions  as  that  of  Krakatoa,  all  of  it  is  thus  dispersed  and 
none  remains  to  form  lava  flows.  Cindery  fragments  thrown  out 
of  the  vent  are  called  sconce,  while  portions  of  still  liquid  lava  thus 
ejected  will,  on  account  of  their  rapid  rotation,  take  on  a  spheri- 
cal form  and  are  called  volcanic  bombs.  Lapilli  are  smaller, 
rounded  fragments,  and  volcanic  ashes  and  dust  are  very  fine  par- 
ticles, though  with  a  wide  range  of  variation  in  size.  The  term 
ashes  is  so  far  unfortunate,  that  it  implies  combustion,  but  never- 
theless it  accurately  describes  the  appearance  of  these  masses. 

In  the  immediate  neighbourhood  of  the  vent  fragments  of  all 
sizes  accumulate,  but  the  farther  we  get  from  the  volcano,  the 
smaller  do  the  fragments  become.  The  coarser  masses  around  the 
vent  form  a  volcanic  agglomerate,  in  which  the  fragments  are  of  all 
shapes  and  sizes,  heaped  together  without  any  arrangement.  More 
regular  sheets  of  large  angular  fragments  form  volcanic  breccia, 
and  these  may  be  seen  on  a  grand  scale  in  the  Yellowstone  Na- 
tional Park,  and  in  many  other  parts  of  the  Rocky  Mountain 
region.  The  finer  accumulations  of  ash,  formed  at  a  greater  dis- 
tance from  the  vent,  are  roughly  sorted  by  the  air  and  often  quite 
distinctly  divided  into  layers.  The  torrents  of  rain,  which  so  fre- 
quently accompany  volcanic  outbursts,  gather  up  the  finer  parti- 
cles, forming  sheets  and  streams  of  hot  mud,  which  on  drying 
sets  into  quite  a  firm  rock,  called  tuff  or  tufa.  Herculaneum  lies 
buried  under  a  solid"  mass  of  such  tuff  more  than  sixty  feet  deep, 
and  hence  is  much  less  accessible  than  Pompeii,  which  is  covered 
with  scoriae,  lapilli,  and  ashes. 


52  VOLCANOES 

As  volcanoes  so  generally  stand  in  or  near  the  sea,  and  as  the 
lighter  fragments,  such  as  pumice,  often  drift  for  months  upon  the 
water  before  they  sink,  while  the  finer  dust  is  carried  vast  dis- 
tances by  the  wind,  it  would  naturally  be  expected  that  volcanic 
materials  should  have  a  very  wide  distribution  upon  the  sea- 
bottom.  Such,  indeed,  proves  to  be  the  case,  -and  this  kind  of 
material,  laid  down  in  the  sea,  has  formed  important  rock-masses 


FIG.    16.  —  Pompeii,  showing  depth  of  volcanic  accumulations.     (Photograph  by 

McAllister.) 

in  nearly  all  the  recorded  ages  of  the  earth's  history.  The  exact 
character  of  the  rock  formed  in  this  fashion  will  be  governed  by 
various  circumstances,  such  as  the  fineness  and  abundance  of  the 
material,  whether  it  is  showered  into  quiet  waters  or  along  a  wave- 
beaten  coast,  whether  and  in  what  proportion  it  is  mingled  with 
sand  or  mud.  When  the  volcanic  ash  preponderates,  a  tuff  is 
formed,  very  much  like  those  which  accumulate  on  land,  but  more 
regularly  stratified. 

The  fragmental  volcanic  products,  whether  coarse  or  fine,  retain 


GASEOUS   PRODUCTS  53 

their  characteristic  texture  and  appearance,  so  as  to  be  readily 
recognizable,  though  perhaps  only  with  the  microscope.  The 
great  bulk  of  these  materials  consists  of  lava  shattered  by  the 
steam  explosions  and  quickly  chilled.  The  coarser  fragments 
display  the  frothy  and  vesicular  nature  of  scoriae,  while  the  finer 
particles  are  glassy  or  crystalline.  Mere  comminution  of  the  mass 
does  not  change  its  essential  texture. 

It  will  be  readily  imagined  that  lavas  do  not  contain  fossils. 
Though  the  flows  often  overwhelm  living  beings,  the  intense  heat 
at  once  destroys  them,  leaving  not  a  trace  behind.  In  tuffs,  on 


FlG.  17.  —  Mauna  Loa,  seen  from  a  distance  of  40  miles.    (Photograph  by  Libbey.) 

the  other  hand,  fossils,  especially  those  of  plants,  are  frequently 
well  preserved,  and  tuffs  formed  under  water  have  fossils  as 
abundantly  as  any  other  aqueous  rocks. 

(3)  The  Gaseous  Products  are  important  as  agents  of  the 
eruptions,  in  promoting  the  crystallizing  of  the  lavas,  and  in  alter- 
ing the  rocks  with  which  they  come  in  contact.  The  most  abun- 
dant is  steam.  Carbon  dioxide  is  common,  especially  when  the 
action  is  failing,  and  often  continues  after  all  other  signs  of  activity 
have  died  out.  Sulphurous  acid  (SO2)  is  very  characteristic  and 
is  the  source  of  many  other  compounds.  Sulphuretted  hydrogen 
(H2S)  is  a  common  volcanic  gas,  as  is  also  hydrochloric  acid 
(HC1).  Several  solids  are  vapourized,  such  as  the  chlorides  of 
ammonium,  iron,  calcium,  etc.,  but  these  are  of  little  significance. 


54 


VOLCANOES 


Volcanic  Cones  are 

built  up  by  the  ma- 
terial which  the  volca- 
noes eject,  and  vary 
in  shape  according  to 
the  character  of  those 
materials  and  to  the 
violence  of  the  erup- 
tions. Those  vents 
which  yield  only  lavas 
build  up  cones  of  solid 
rock,  the  steepness  of 
which  corresponds  to 
the  degree  of  fluidity 
of  the  flows.  The  re- 
markably liquid  lavas 
of  the  Sandwich  Isl- 
ands have  formed 
cones  of  exceedingly 
gentle  slope,  3°  to 
10°  (see  Fig.  17,  the 
cone  of  Mauna  Loa). 
Very  stiff  lavas  which 
consolidate  rapidly 
form  very  steep-sided 
cones.  The  cones 
which  are  constructed 
principally  out  of 
fragmental  materials 
are  steep  (30°);  the 
more  so,  the  coarser 
the  fragments  which 
compose  them,  and 
often  beautifully  sym- 
metrical, as  in  the 
noble  mountains  of 


SUBMARINE  VENTS  55 

our  Pacific  States,  such  as  Mt.  Shasta,  Mt.  Hood,  and  Mt.  Rainier. 
Most  cones  are  built  up  of  scoriae,  ashes,  and  lava  flows,  while 
the  fissures  that  radiate  from  the  crater  are  filled  by  dikes, 
greatly  strengthening  the  mountain,  as  in  the  case  of  Vesuvius. 
The  latter  is  noted  for  its  double  head,  Monte  Somma  being 
part  of  the  old  cone  which  was  mostly  blown  away  in  the  erup- 
tion of  79  A.D. 


FlG.  19.  —  Vesuvius  and  Monte  Somma. 

Volcanoes,  like  other  mountains,  are  subject  to  the  destructive 
activity  of  the  atmosphere,  of  rivers  and  of  the  sea,  and,  when 
eruptions  have  ceased,  this  destruction  may  go  on  with  great 
rapidity,  especially  in  the  case  of  cones  made  up  of  loose  materials. 
Very  ancient  cones  can  seldom  be  found/for  this  reason,  and  often 
the  lava-filled  pipe  is  the  only  record  left  of  an  ancient  volcano. 

Submarine  Volcanoes  have  been  actually  observed  in  eruption 
in  several  instances,  but  there  can  be  little  doubt  that  by  far 
the  larger  number  have  escaped  detection.  Not  a  few  volcanic 
islands  in  various  parts  of  the  world  have  reared  themselves 


56  VOLCANOES 

above  the  sea  within  historic  times.  In  fact,  all  volcanic  islands 
are  merely  submarine  volcanoes,  or  groups  of  them,  which  have 
built  their  cones  above  sea-level.  If  the  cones  are  of  fragmental 
products,  the  islands  are  but  temporary,  because  when  the  activity 
ceases,  the  waves  cut  them  down  into  reefs  and  shoals.  The  lava 
cones  persist  for  long  periods. 


FlG.  20.  —  Truncated  tuff  cone,  island  ofOahu.     (Photograph  by  Libbey.) 

Fissure  Eruptions.  —  There  is  much  reason  to  believe  that  the 
mode  of  volcanic  eruption  from  a  single  vent,  described  in  the 
foregoing  pages,  is  not  the  only  method  by  which  molten  lava 
may  reach  the  surface.  It  would  seem  that  in  past  times  lava  has 
welled  up  through  great  fissures  and  overflowed  immense  areas  in 
successive  floods.  As  an  example  of  this  may  be  mentioned  the 
vast  fields  of  lava  which  occur  in  the  northwestern  United  Stater, 
covering  more  than  100,000  square  miles  to  the  depth  of  several 
hundred  feet.  The  largest  connected  area  of  this  field  extends 
along  the  Snake  River  in  Idaho;  southwest  from  the  Yellowstone 


FISSURE  ERUPTIONS 


57 


. 

SI 


II 


58  VOLCANOES 

Park.  Even  more  extensive  are  the  lava  plains  of  the  Deccan  in 
India,  and  much  smaller,  but  still  impressive,  fields  occur  in  Ire- 
land and  Scotland. 

The  Causes  of  Volcanic  Activity.  —  Many  theories  have  been 
advanced  to  explain  the  causes  of  volcanic  activity,  but  none  are 
satisfactory.  In  an  elementary  work,  like  the  present,  no  ade- 
quate discussion  of  this  most  difficult  problem  can  be  given,  but 
merely  a  brief  sketch  of  some  of  the  ways  in  which  its  solution 
has  been  attempted. 

The  problem  is  to  account  (i)  for  the  intense  heat  of  the 
ejected  materials,  (2)  for  the  presence  of  the  steam,  (3)  for 
the  ascensive  force  of  the  lava,  and  (4)  for  the  intermittency 
of  the  action,  and  the  past  and  present  distribution  of  the 
vents. 

(i)  The  high  temperature  has  been  accounted  for  in  two  prin- 
cipal ways.  By  some  it  is  supposed  that  it  is  due  to  the  original 
heat  of  the  earth,  not  yet  lost  by  radiation.  Of  those  who  accept 
this  opinion,  some  believe  that  larger  or  smaller  portions  of  the 
earth's  interior  have  never  solidified  and  that  these  form  the 
reservoirs  of  lava  which  supply  the  volcanic  vents.  Others  again 
assume  that  the  interior  of  the  globe  is  exceedingly  hot,  but  solidi- 
fied by  pressure ;  when,  by  fracturing  or  folding  of  the  overlying 
rocks,  this  pressure  is  partially  relieved,  the  highly  heated  masses 
become  liquefied  along  that  line  of  reduced  pressure. 

In  the  second  class  of  hypotheses  on  this  subject  of  tempera- 
ture, it  is  supposed  that  the  proper  heat  of  the  earth's  interior  is 
no  longer  sufficient  to  produce  fusion,  and  that  it  must  be  sup- 
plemented from  some  other  source.  Many  are  the  attempts  to 
determine  where  this  additional  source  of  heat  supply  is  to  be 
found.  One  of  the  most  celebrated  of  these  attempts  (Mallet) 
seeks  the  additional  heat  in  the  friction  produced  by  the  folding 
and  crushing  of  rocks  deep  within  the  crust  of  the  earth,  due 
to  the  shrinkage  of  the  earth  as  it  cools.  Others  have  sought  to 
show  that  the  heat  is  generated  chemically,  by  the  oxidizing  effect 
of  descending  waters  upon  the  unoxidized  interior  of  the  globe, 
or  by  the  combustion  of  hydrogen  gas. 


CAUSES   OF   VOLCANIC  ACTIVITY  59 

Similar  divergences  of  opinion  obtain  with  regard  to  the  nature 
and  origin  of  the  lavas  ejected  by  volcanoes.  The  view  most 
commonly  held  is  that  they  are,  for  the  most  part,  the  original, 
unaltered  material  of  the  globe,  whether  this  has  always  remained 
fluid,  or  has  been  remelted  by  release  of  pressure,  or  otherwise. 
According  to  another  opinion,  volcanic  products  are  formed  from 
the  fusion  of  sedimentary  material  which  was  laid  down  under 
water,  but  has  been  deeply  buried  within  the  crust  of  the  earth  by 
subsidence.  A  third  view  recognizes  both  sources  of  supply. 

(2)  The  problem  as  to  the  origin  of  the  steam  which  plays  so 
important  a  part  in  volcanic  eruptions,  is  likewise  very  differently 
solved  by  different  investigators.     One  opinion  is  that  the  steam, 
like  the  lava  itself,  is  primordial  and  was  absorbed  from  the  atmos- 
phere (which  then  contained  all  the  waters  of  the  sea)  when  the 
surface  of  the  globe  was  still  molten.     Melted  substances  will,  it 
is  known,  absorb   many  times  their  own  volume  of  steam  and 
gases,  when  in  contact  with  them  under  pressure.     From  this  it  is 
inferred  that  the  lava  has  contained  the  steam  ever  since  the  first 
cooling  of  the  surface  crust.     A  second  opinion  derives  the  water 
from  the  surface  of  the  earth,  supposing  that  it  descends  partly 
through  fissures  and   partly  through  the   pores  of  the  overlying 
rocks   by  capillarity.      The  nearness  of  volcanoes   to   the   sea  is 
•looked  upon  as  favouring  this  view.     Others,  again,  employ  both 
methods  of  explanation,  regarding  the  ordinary  steam  which  im- 
pregnates all  lavas  as  primordial,  but  believing  that  the  violently 
explosive  eruptions  are  caused   by  the  sudden  access  of  large 
bodies  of  water  to  the  lava  masses. 

(3)  The  causes  of  the  ascensive  force  of  the  lava  column  are 
likewise  very  differently  explained  by  various  writers.     Some  find 
an  all-sufficient  cause  in  the  steam  pressure,  while  others  maintain 
that  some  other  force  must  be  at  work  and  find  this  in  the  unequal 
contraction  of  the  earth,  and  consequent  pressure  upon  the  molten 
or  plastic  layer  beneath.     It  has  been  calculated  that  a  radial  con- 
traction of  one  millimetre  "  would   suffice  to   supply  matter  for 
five  hundred  of  the  greatest  known  volcanic  eruptions"  (Prest- 
wich) . 


60  VOLCANOES 

(4)  The  intermittency  of  volcanoes  and  their  mode  of  distribu- 
tion add  to  the  difficulty  of  the  whole  subject,  but  any  complete 
theory  must  explain  them.  The  views  which  bring  volcanic  action 
into  relation  with  the  mechanical  changes  in  the  crust,  are  those 
which  seem  most  consonant  with  the  known  facts  of  the  past  and 
present  distribution  of  the  vents. 

Here,  for  lack  of  space,  we  must  leave  the  subject.  Enough 
has  been  said  to  show  how  far  we  still  are  from  understanding  the 
mystery  of  volcanoes. 


CHAPTER  III 

-^ 

EARTHQUAKES  —  CHANGES  OF  LEVEL 

AN  earthquake  consists  of  a  series  of  elastic  waves,  similar  in 
principle  to  sound  waves,  propagated  through  the  crust  of  the 
earth  and  due  to  some  disturbance  in  its  interior.  The  number 
and  frequency  of  earthquakes  are  exceedingly  great,  30  much  so, 
that  the  crust  of  the  earth  is  in  a  state  of  constant  disturbance  at 
one  or  more  points.  Besides  the  trembling  and  shocks  that  may 
be  plainly  felt,  minute  tremors,  which  can  be  detected  only  by  the 
aid  of  very  delicate  instruments,  are  in  continual  progress,  even  in 
countries  rarely  visited  by  sensible  shocks.  Slight  shocks  and 
movements  generally  pass  unnoticed,  but  when  pains  are  taken  to 
collect  them,  it  is  surprising  to  find  how  numerous  they  are.  These 
facts  show  that  the  crust  of  the  earth  is  not  the  stable,  rigid  structure 
which  our  ordinary  experience  would  lead  us  to  consider  it. 

The  Distribution  of  Earthquakes  is  very  similar  indeed  to  that 
of  volcanoes,  and  volcanic  regions  are  preeminently  those  in  which 
the  most  frequent  and  violent  earthquakes  occur.  They  may 
manifest  themselves  at  any  part  of  the  earth's  surface,  but  they 
increase  notably  both  in  frequency  and  violence  as  the  volcanic 
areas  are  approached.  The  great  earthquakes  which  shook  the 
Mississippi  valley  in  1811-12,  are  among  the  very  few  instances 
of  violent  and  long-continued  shocks  in  a  region  far  from  any  vol- 
cano. Yet,  even  these  would  appear  to  have  had  some  connection 
with  the  volcano  of  St.  Vincent  in  the  West  Indies,  and  ceased 
when  that  vent  became  active. 

The  earthquake,  or  seismic^  bands  thus  follow  the  coast-lines 
and  mountain  chains,  and  coincide  with  the  volcanic  bands,  but 
are  much  wider  than  the  latter,  earthquakes  being  propagated  over 

1  Greek  seismos,  earthquake. 

61 


62  EARTHQUAKES 

wider  areas  from  the  seat  of  disturbance  than  the  effects  of  vol- 
canoes, except  in  the  case  of  the  unusually  terrible  explosions. 
In  one  respect,  the  earthquake  bands  deviate  quite  markedly  from 
the  volcanic ;  namely,  in  the  presence  of  a  seismic  zone  which 
encircles  the  whole  earth.  This  belt  includes  the  Mediterranean 
lands,  the  Azores,  the  West  Indies  and  Central  America,  the  Sand- 
wich Islands,  Japan,  China,  India,  Persia,  and  Asia  Minor.  By 
some  authorities  this  is  regarded  as  the  main  seismic  band,  espe- 
cially liable  to  disturbance,  from  which  the  others  are  but  branches. 
Volcanoes  occur  in  many  parts  of  the  belt,  but  they  form  no  such 
continuous  band  around  the  earth. 

The  Phenomena  of  Earthquakes  are  of  very  great  significance 
from  the  standpoint  of  human  interests,  because  of  the  appalling 
destruction  and  loss  of  life  which  they  often  cause.  Fearful  re- 
sults may  be  caused  by  a  heaving  of  the  ground  in  which  there 
is  little  actual  displacement.  The  phenomena  vary  considerably, 
according  to  the  position  of  the  place  observed  with  reference 
to  the  focus,  or  seat  of  disturbance,  the  violence  of  the  shock, 
and  the  character  of  the  rocks  through  which  the  vibrations  are 
transmitted.  Were  the  focus  a  point,  and  the  rocks  uniformly 
elastic  and  homogeneous,  the  earthquake  waves  would  be  spheri- 
cal, and  their  outcroppings  at  the  surface  (which  produce  the  sen- 
sible shocks)  would  be  circular.  But  none  of  these  conditions 
exist,  and  so  the  waves  are  more  or  less  irregular,  and  owing  to 
differences  in  the  rocks,  reflection  and  interference  of  the  waves, 
and  the  like,  the  result  often  seems  most  anomalous  and  capri- 
cious. This,  however,  is  not  of  sufficient  importance  for  our  pur- 
pose to  require  description. 

When  the  shock  occurs  .beneath  the  bed  of  the  sea,  the  phe- 
nomena are  complicated  by  disturbances  in  the  water.  Of  these, 
the  most  important  is  the  great  sea  wave  (erroneously  called  tidal 
wave),  which,  though  not  strikingly  displayed  in  the  open  sea,  in 
shallow  water  piles  up  into  enormous  breakers  and  rushes  upon  the 
coast,  often  doing  far  more  damage  than  the  earth  waves  themselves. 

One  important  result  of  the  modern  careful  investigation  of 
earthquakes  is  to  show  that  they  are  comparatively  superficial  phe- 


EFFECTS   OF   EARTHQUAKES  63 

nomena,  which  arise  within  and  not  below  the  crust  of  the  earth. 
The  focus  is  usually  situated  at  depths  of  eight  or  ten  miles,  while 
depths  of  twenty-five  to  thirty  miles  are  uncommon. 

The  Effects  of  Earthquakes  are  geologically  less  important  than 
is  usually  supposed,  and  some  of  them  which  are  commonly  called 
effects  of  earthquakes,  are  rather  collateral  results  of  the  forces 
which  produced  the  earthquake.  The  heaving  and  wave-like  roll- 
ing of  the  ground,  when  violent,  bring  about  great  landslips  in 
mountain  regions,  and  shake  down  enormous  masses  of  earth  and 
rock  from  the  cliffs  and  slopes,  into  the  valleys  beneath.  Destruc- 
tion of  this  character  on  a  gigantic  scale  was  a  marked  feature  of 
the  great  earthquakes  which  shook  northwestern  Greece  in  1870. 
These  falling  masses  may  temporarily  or  permanently  block  up  the 
mouth  of  a  mountain  valley,  and  by  damming  back  the  stream 
which  flows  in  the  valley,  convert  it  into  a  lake.  The  course  of 
rivers  is  not  unfrequently  altered  by  earthquakes,  a  ridge  being 
thrown  up  athwart  the  stream.  Along  the  west  coast  of  South 
America  are  found  many  deserted  stream  channels,  the  streams 
having  evidently  been  diverted  by  the  upheaval  of  ridges  across 
their  courses.  In  a  narrow  valley  such  a  ridge  will  act  as  a  dam 
and  form  a  lake,  but  on  a  plain  the  stream  will  be  diverted  to 
a  new  course. 

A  very  common  accompaniment  of  violent  earthquakes  is  the 
opening  of  cracks  and  fissures  in  the  ground ;  these  may  close  up 
again  after  the  shock  has  passed,  but  they  often  remain  open  as 
yawning  chasms.  The  two  sides  of  such  a  fissure  may  remain 
at  the  same  relative  level  as  before,  or  one  side  may  be  raised  up 
or  dropped  down,  producing  a  dislocation  or  fault.  The  earth- 
quake which  originated  in  Owen's  Valley,  California,  in  1872,  and 
was  one  of  the  most  violent  recorded  within  the  United  States, 
was  accompanied  by  a  series  of  faults,  running  along  the  base  of 
the  Sierra  Nevada,  and  having  a  maximum  displacement  of  twenty 
feet.  In  May,  1887,  an  earthquake  in  Arizona  and  Sonora  (Mex- 
ico) was  accompanied  by  a  fault,  which  has  been  traced  for  thirty- 
five  miles,  and  has  an  average  displacement  of  seven  feet.  Similar 
phenomena  have  been  recorded  from  many  parts  of  the  world. 


64  EARTHQUAKES 

The  Causes  of  Earthquakes  are  better  understood  than  those  of 
volcanoes,  though  much  still  remains  which  is  difficult  of  explana- 
tion. All  earthquakes  are  not  due  to  the  same  causes,  for  any 
operation  which  will  produce  a  blow  or  jar  in  the  earth's  interior 
sufficiently  strong  to  be  propagated  as  a  series  of  elastic  waves  to 
the  surface,  will  cause  an  earthquake.  The  most  frequent  of  such 
sources  of  disturbance  are  of  two  kinds. 

(1)  In  volcanic  regions  the  explosions  of  steam  are  very  often 
the  cause  of  earthquakes.     This  explains  the  association  of  earth- 
quakes and  volcanoes  in  the  same  region,  and  also  the  fact  that  a 
great  volcanic  eruption  is  usually  heralded  by  earthquakes,  which 
increase  in  violence  up  to  the  time  of  the  outbreak,  and  then 
cease.     Though  more  than  two  thousand  miles  distant,  the  erup- 
tion of  St.  Vincent  relieved  the  Mississippi  valley  earthquakes.     It 
must  not  be  supposed,  however,  that  all  the  earthquakes  which 
occur  in  volcanic  areas  can  be  brought  into  relation  with  eruptions, 
for  many  of  them  cannot,  as  in  the  case  of  the  countries  around 
the  Mediterranean  and  of  Japan. 

(2)  A  second  and  probably  more  important  and  widespread 
cause  of  earthquakes  is  the  sudden  yielding  of  the  earth's  crust 
to  the  strains  which  are  set  up  within  it.     The  contraction  of  the 
earth  must  establish  cumulative  stresses  in  the  crust,  which,  if 
yielded  to  gradually,  would  not  cause  a  jar,  but  when  resisted, 
increase  until  the  strength  of  the  rocks  is  overcome,  and  they  give 
way  with  a  sudden  shock,  which  produces  an  earthquake.     The 
association  of  volcanoes  and  earthquakes  in  the  same  regions  is 
thus  not  altogether,  probably  not  even  principally,  a  relation  of 
cause  and  effect,  but  is  rather  due  to  the  fact  that  both  are  con- 
nected with  lines  of  fracture  and  weakness  in  the  earth's  crust. 
In  non-volcanic  regions  nearly  all  the  earthquakes  may  be  traced 
to  such  lines  of  fracture,  wherever  the  data  have  been  collected 
for  an  accurate  determination  of  the  foci. 

CHANGES  OF  LEVEL 

As  was  mentioned  in  the  preceding  section,  permanent  changes 
of  level  frequently  accompany  earthquakes;  but  these  are  par- 


CHANGES   OF   LEVEL  65 

oxysmal,  and  appear  to  be  nearly  always  the  result  of  dislocation 
or  faulting.  By  change  of  level,  in  the  general  sense,  is  meant  the 
gradual,  steady  upheaval  or  subsidence  of  land,  with  reference  to 
the  sea,  over  considerable  areas.  Such  movements  are  very  slow, 
and  hence  are  apt  to  escape  observation ;  so  long  are  the  periods 
of  time  involved,  that  there  is  much  dispute  as  to  the  facts,"  and 
still  more  as  to  the  interpretation  of  them. 

The  method  by  which  any  change  in  the  relative  position  of  the 
land  may  be  detected,  is  by  comparison  with  the  sea,  and  hence 
such  observations,  so  far  as  they  refer  to  changes  now  in  progress, 
are  confined  to  the  coast.  Obviously,  the  result  would  be  the 
same  if  the  change  were  in  the  sea,  and  it  is  somewhat  doubtful 
which  is  to  be  regarded  as  the  seat  of  a  given  oscillation.  Until 
quite  lately  it  was  always  taken  for  granted  that  the  sea-level  is 
constant,  and  that  the  surface  of  the  oceans  is  everywhere  that  of 
a  true  spheroid,  concentric  with  the  figure  of  the  earth.  Recent 
exact  investigations  have,  however,  thrown  much  doubt  upon  this 
assumption,  and  indicated  that  the  sea-level  may  be  markedly 
different  at  different  places.  By  this  is  not  meant  the  temporary 
change  due  to  the  heaping  up  of  the  waters  by  the  wind,  or  by 
unusual  tides,  but  a  real  and  permanent  difference  of  average 
level.  There  is  reason  to  believe  that  the  attraction  of  the  con- 
tinents, of  great  mountain  ranges  and  lofty  plateaus  near  the  coast, 
raises  the  water  to  a  higher  level  than  in  the  open  sea,  or  along 
flat,  low-lying  shores. 

These  local  differences  greatly  complicate  the  problem  of  deter- 
mining just  what  processes  are  at  work  in  the  apparent  elevation 
and  depression  of  land.  For  this  reason  some  geologists  have 
proposed  to  avoid  the  terms  elevation  and  depression,  and  to 
substitute  for  them  "  positive  and  negative  displacements  of  the 
coast-line,"  which  are  non-committal  with  reference  to  the  real 
character  of  the  movement.  For  the  sake  of  convenience,  it  will 
be  best  to  retain  the  older  and  more  current  terms,  without  insist- 
ing that  in  all  cases  the  changes  lie  in  the  land  rather  than  in 
the  sea. 

Another  process  which  must  be  carefully  distinguished  from  the 


66  CHANGES   OF   LEVEL 

true  changes  of  level  is  the  alteration  of  the  coast-line  due  to  the 
washing  away  of  land  by  the  sea,  causing  the  latter  to  advance,  or, 
on  the  other  hand,  to  the  silting  up  of  shallow  water  by  the  depo- 
sition of  sand  or  mud,  which  makes  the  land  advance  at  the  ex- 
pense of  the  sea. 

The  evidences  of  elevation  along  any  coast-line  may  be  more 
easily  made  out  than  those  of  depression,  because  when  land 
comes  up  out  of  the  sea,  the  traces  of  marine  action  upon  its 
surface  are  always  present  in  one  form  or  another.  When,  on  the 
contrary,  land  goes  down  beneath  the  sea  or  the  sea  rises  over 
the  land,  the  old  land  surface  is  speedily  changed  and  buried  out 
of  sight. 

Evidences  of  Elevation.  —  Along  coasts  which  have  been  inhab- 
ited for  many  centuries  by  civilized  man,  ancient  structures,  like 
quays  and  bridges,  which  were  built  in  the  water  may  now  be 
found  high  above  it.  Such  changes  have  been  noted  in  the  lands 
which  adjoin  the  Mediterranean,  especially  in  southern  Italy  and 
the  island  of  Crete.  The  temple  of  Jupiter  Serapis  at  Puzzuali, 
near  Naples,  is  a  classic  example  of  this.  In  this  building  thiee 
marble  columns  are  still  standing,  and  on  each  of  them  is  a  belt, 
about  ten  feet  above  the  ground  and  nine  feet  wide,  honey-combed 
by  the  boring  mollusc,  Lithodomus,  which  still  lives  in  the  neigh- 
bouring bay.  Evidently  the  temple  was  first  submerged,  and  when 
under  water  the  columns  were  attacked  by  the  mollusc,  and  after- 
wards it  was  upheaved,  but  is  now  sinking  once  more. 

Rocks  and  cliffs  long  exposed  to  the  action  of  the  surf  are  worn 
and  marked  in  a  characteristic  fashion,  and  when  found  far  above 
where  the  surf  can  now  reach  them,  are  evidences  of  upheaval  at 
that  point.  Such  well-defined  sea-marks  high  above  sea-level  are 
common  in  the  high  latitudes  of  the  northern  hemisphere,  and  the 
change  is  still  in  progress  in  many  places.  The  Scandinavian 
peninsula  shows  slow  changes  of  level,  though  not  everywhere  the 
same  ;  south  of  Stockholm  there  is  a  slight  subsidence,  north  of  that 
point  there  is  a  rise,  which  increases  northward,  and  at  the  North 
Cape  is  believed  to  average  five  or  six  feet  per  century.  These 
results  have  been  much  questioned,  but  after  a  renewed  examina- 


EVIDENCES   OF  DEPRESSION  6/ 

tion  of  the  dated  tide  marks  cut  on  the  cliffs  at  various  periods  in 
the  last  century  and  early  in  the  present  one,  the  Swedish  geolo- 
gists have  strongly  reaffirmed  them. 

Interesting  examples  of  recent  elevation  are  believed  to  occur 
in  the  neighbourhood  of  Washington,  D.C.  In  colonial  times 
Bladensburg  and  Dumfries  could  be  reached  by  sea-going  ships, 
but  now  they  are  decidedly  above  tide-level.  The  change  is 
generally  supposed  to  be  due  to  a  silting  up  of  the  creeks,  but 
this  appears  not  to  be  the  case,  for  there  is  little  alluvium  resting 
upon  the  bed-rock  of  the  channels. 

In  many  parts  of  the  world  are  found  beaches,  raised  far  above 
the  sea-level,  and  these  point  to  an  elevation  of  the  land.  Such 
raised  beaches  occur  in  Scandinavia,  Great  Britain,  the  West 
Indies,  the  Red  Sea,  the  southern  end  of  South  America,  and 
elsewhere.  In  these  beaches  are  often  found  the  remains  of 
marine  animals,  shells,  corals,  barnacles,  and  the  like,  and  where- 
ever  such  remains  occur  in  undisturbed  position,  they  prove  con- 
clusively either  that  the  land  has  been  upheaved  or  that  the  sea 
has  gone  down  at  that  point.  The  form  and  character  of  the 
coast-line  itself  may  be  evidence  of  comparatively  recent  eleva- 
tion, as  will  be  explained  in  Part  III. 

Evidences  of  Depression.  —  Ancient  buildings,  which  were  evi- 
dently constructed  on  land,  but  now  are  below  the  sea,  show  a 
subsidence  at  the  point  where  they  occur.  These  are  found  in 
several  of  the  Mediterranean  lands  and  on  the  west  coast  of 
Greenland.  In  the  latter  country  the  change  is  relatively  rapid, 
and  has  attracted  the  attention  of  the  inhabitants. 

Buried  forests  found_  Jbglow  the  level  of  the  sea  indicate  subsid- 
ence. Such  forests  occur  in  the  Mississippi  delta  and  at  many 
points  on  the  coast  of  the  middle  and  southern  Atlantic  States, 
notably  in  New  Jersey,  where  the  coast  is  sinking  at  a  rate  esti- 
mated as  two  feet  per  century. 

jjubmerged  j:jver-channels_are  likewise  evidences  of  depression, 
because  a  river  which  flows  into  the  sea  cannot  excavate  its  bed 
below  the  level  of  the  sea-bottom  at  its  mouth.  Many  such  in- 
stances are  known,  but  it  will  suffice  to  mention  the  case  of  the 


68  CHANGES   OF  LEVEL 

Hudson,  whose  ancient  channel  has  been  traced  by  means  of 
soundings  for  one  hundred  miles  outside  of  Sandy  Hook. 

A  great  thickness  of  shallow-water  deposits  is  another  obvious 
proof  of  depression ;  for  if  the  sea- bottom  did  not  sink,  the  water 
would  soon  be  filled  up  where  it  is  shallow,  and  the  coast-line 
advanced.  When  we  come  to  study  the  materials  which  are  now 
accumulating  on  the  ocean  floor,  we  shall  learn  how  the  depth  of 
water  may  be  inferred  from  the  character  of  the  deposits.  At 
present  it  is  only  necessary  to  say  that,  from  the  Hudson  River 
southward,  the  Atlantic  coastal  plain  is  covered  by  a  great  thick- 
ness of  shallow-water  beds,  as  revealed  by  the  numerous  artesian 
wells  which  have  been  driven  through  it.  A  coast  may  also  betray 
recent  subsidence  by  its  form,  but  an  account  of  this  must  be 
deferred  till  we  reach  Part  III. 

The  Causes  of  Elevation  and  Depression.  —  Perplexing  and 
obscure  as  we  have  found  the  causes  of  the  other  subterranean 
agencies  to  be,  those  which  control  the  changes  of  level  are  far 
more  so,  and  no  theory  of  their  action  yet  propounded  is  at  all 
adequate  or  satisfactory.  Perhaps  the  most  generally  accepted 
view  is  that  which  brings  them  into  relation  with  the  secular  con- 
traction of  the  earth,  due  to  the  slow  shrinkage  of  the  heated 
interior,  as  it  gradually  cools,  together  with  the  flow  of  plastic 
subcrustal  material  away  from  the  lines  of  greatest  pressure.  This 
theory  is,  however,  very  vague,  and  cannot  be  crucially  tested. 

Another  kind  of  movement  of  the  crust  of  the  earth  is  believed 
to  h£L.that  which  follows  upon  changes  of  the  load  which  any  given 
area  has  to  carry.  We  shall  see  in  later  chapters  that  the  rain, 
wind,  and  rivers  are  continually  cutting  down  the  land  surface  and 
carrying  the  materials  thus  obtained  to  the  sea,  where  they  accu- 
mulate in  great  sheets.  By  some  authorities  it  is  believed  that  the 
crust  of  the  earth  is  in  so  exact  a  state  of  equilibrium,  that  any 
area  upon  which  a  load  of  sediment  is  deposited  will  sink,  while 
one  whence  material  is  removed  will  rise  because  its  load  is  light- 
ened. This  kind  of  adjustment  is  called  isostasy,  and  its  effect  is 
to  preserve  the  inequalities  of  the  earth's  surface  as  they  already 
exist,  while  the  general  movements  of  elevation  and  depression 


SUMMARY   OF    IGNEOUS   AGENCIES  69 

must  be  quite  independent  of  isostatic  adjustment,  because  they 
increase  or  diminish  those  inequalities. 

The  most  recent  investigations  are  unfavourable  to  this  view 
of  isostasy,  and  go  to  show  that  the  crust  is  not  so  sensitive  to 
changes  of  load.  While  the  major  inequalities,  the  great  conti- 
nental platforms  and  oceanic  depressions,  are  not  improbably  due 
to  differences  of  density  in  the  crust,  minor  inequalities,  like  hills 
and  mountains,  are  apparently  sustained  by  the  rigidity  of  the  crust. 

Summary. — The  study  of  the  subterranean,  or  igneous,  agen- 
cies has  proved  to  be  very  unsatisfactory  in  the  way  of  explaining 
the  phenomena  and  referring  them  to  the  operation  of  understood 
physical  agents,  because  so  little  is  really  known  and  so  much 
remains  to  be  discovered.  Nevertheless,  we  have  learned  much 
that  is  of  great  importance  in  geological  reasoning.  We  have 
seen  that  the  earth  contains  within  itself  a  great  store  of  energy, 
and  that  its  interior,  in  whatever  physical  state  that  may  be,  is 
highly  heated,  and  possesses  great  quantities  of  material  which  is 
either  actually  or  potentially  molten,  and  is  permeated  with  super- 
heated steam  and  other  gases.  This  molten  material  is  often 
forced  upward,  and  is  either  poured  out  at  the  surface,  or  fills  up 
fissures  and  cavities  in  the  rocks,  or  pushes  its  way  between  them. 
Cooling  under  various  circumstances,  the  molten  masses  consoli- 
date into  a  great  variety  of  characteristic  rocks,  frothy,  glassy,  or 
crystalline.  Explosive  discharges  of  steam  blow  the  melted  rock 
into  fragments  of  all  grades  of  fineness,  and  these  fragments  like- 
wise accumulate  either  on  the  land  or  under  water,  and  form 
rocks,  the  nature  and  origin  of  which  may  be  readily  recognized. 

We  have  further  seen  that  the  operation  of  these  subterranean 
forces  produces  shocks  and  jars  in  the  interior,  which  are  propa- 
gated to  the  surface  as  earthquakes,  and  there  bring  about  per- 
manent changes,  associated  with  the  fissuring  and  dislocation  of 
the  rocks,  landslips,  alteration  in  the  course  of  rivers,  formation 
of  lakes,  and  the  like.  The  frequency  of  earthquakes,  their  wide 
geographical  range,  and  the  constant  tremor  of  the  ground 
detected  by  delicate  instruments,  led  us  to  infer  that  the  crust 
of  the  earth  is  decidedly  unstable. 


70  IGNEOUS  AGENCIES 

This  conclusion  we  found  strengthened  by  the  oscillations  of 
level  between  land  and  sea,  which,  though  extremely  slow,  are 
seen  to  be  still  in  progress.  Historical  geology  will  show  us  that 
these  changes  of  level  have,  in  the  course  of  ages,  been  effected 
on  the  grandest  scale.  All  the  great  continents  are  composed  of 
rocks,  which,  for  the  most  part,  were  laid  down  in  the  sea  and 
still  contain  the  fossils  of  marine  animals,  and  this  shows  that 
these  continents  have  all  been  under  the  sea.  Not  that  all  the 
continents,  or  even  that  all  parts  of  any  one  continent,  were  sub- 
merged at  the  same  time,  but  now  one  part  and  now  another  was 
overflowed  and  again  emerged,  until  all  have  been  covered  in  their 
turn. 

In  brief,  the  principal  geological  functions  of  the  subterranean 
agencies  are  two  :  (i)  they  bring  up  from  below  and  form  at  the 
surface,  and  at  all  depths  beneath  it,  certain  characteristic  kinds  of 
rocks  ;  and  (2)  they  tend  to  increase  the  inequalities  of  the  earth's 
surface,  and  thus  to  counteract  the  agencies  which  are  cutting 
down  the  land  and  steadily  tending  to  reduce  it  to  the  level  of 
the  sea. 


SECTION    II 

SURFACE  AGENCIES 


THE  surface  or  superficial  agents  are  those  which  act  upon  or 
very  near  the  surface  of  the  ground  and  all  of  which  are  mani- 
festations of  solar  energy.  Their  work  may  all  be  summed  up  in 
two  categories,  the  destruction  and  reconstruction  of  rock.  These 
two  processes  are  complementary ;  for  since  matter  cannot  be 
destroyed,  but  can  have  only  its  position  and  its  physical  and 
chemical  relations  changed,  it  is  obvious  that  what  is  removed  in 
one  place  must  be  laid  down  in  another.  Thus,  neither  of  these 
processes  can  go  on  without  the  other,  and  reconstruction  always 
implies  antecedent  destruction  to  furnish  the  materials.  Every- 
where we  find  ceaseless  cycles  of  change  in  progress,  new  combi- 
nations of  material  being  continually  formed  and  older  rocks 
steadily  worked  over  into  newer.  It  is  this  circulation  of  matter 
upon  and  within  the  crust  of  the  earth  that  we  have  already  com- 
pared to  the  physiological  changes  in  the  body  of  a  living  organism. 

It  is  important  to  remember  that  the  processes  of  rock  destruc- 
tion, which  are  grouped  together  under  the  general  name  of 
denudation  or  erosion,  are  confined  almost  entirely  to  the  land 
surfaces,  while  those  of  reconstruction  take  place  principally 
beneath  bodies  of  water.  Some  work  of  reconstruction  is  also 
accomplished  on  the  land,  but  this  is  of  very  minor  importance. 

Since  destruction  necessarily  precedes  reconstruction,  the  con- 
sideration of  these  processes  will  naturally  begin  with  those  of 
destruction  or  denudation.  The  destructive  agencies  are  :  (i)  the 
atmosphere,  (2)  running  water,  (3)  ice,  (4)  lakes,  (5)  the  sea, 
(6)  plants  and  animals. 

71 


CHAPTER    IV 
DESTRUCTIVE  PROCESSES  — THE  ATMOSPHERE 

THE  atmospheric  agencies  are  by  far  the  most  important  of  the 
destructive  or  denuding  agents,  because  no  part  of  the  land  surface 
is  altogether  exempt  from  their  activity.  Their  work  is  described 
by  the  general  term  weathering,  and  is  shown  at  once  by  the 
different  appearance  of  freshly  quarried  stone  from  that  which  has 
been  long  exposed  in  the  face  of  a  cliff,  or  even  in  ancient  build- 
ings. While  such  agents  as  rivers  and  the  sea  do  work  that  is 
much  more  apparent  and  striking  than  that  of  the  atmosphere, 
yet  they  are  much  more  locally  confined,  and  even  in  their  opera- 
tions the  atmosphere  renders  important  aid.  Though  no  part 
of  the  land  surface  is  entirely  free  from  the  destructive  activity  of 
the  atmosphere,  the  rapidity  and  intensity  of  this  activity  vary 
much  in  different  places.  There  are,  in  the  first  place,  the  great 
differences  of  climate  to  be  considered,  differences  in  the  amount 
and  distribution  of  the  rainfall,  of  temperature,  and  of  the  winds. 
In  the  second  place,  the  resistance  offered  by  the  various  kinds 
of  rocks  to  the  disintegrating  processes  differs  very  greatly,  in 
accordance  with  the  differences  of  hardness  and  chemical  compo- 
sition. Again,  the  presence  or  absence  of  a  covering  of  protective 
vegetation  has  an  important  influence  upon  the  amount  and  char- 
acter of  the  destruction  effected. 

The  outcome  of  all  these  varying  factors  is  to  produce  very 
irregular  land  surfaces.  While  the  tendency  of  the  atmospheric 
agencies  is  gradually  to  wear  down  the  land  to  the  level  of  the 
sea,  yet  in  that  process  some  parts  are  cut  away  much  more  rapidly 
than  others;  and  hence  the/™/  effect  of  denudation  is  an  increas- 
ingly irregular  surface.  The  overlying  screen  of  Goil  conceals  many 
of  these  irregularities,  especially  the  minor  ones ;  and  were  that 

72 


RAIN 


73 


screen  removed,  the  ruggedness  of  the  underlying  rocks  would  be 
seen  to  be  much  more  decided  than  now  appears.  So  long  as 
the  land  has  these  irregularities,  it  is  said  to  possess  relief ;  and 
when  it  is  all  planed  down  to  a  flat  or  gently  sloping  surface  but 
slightly  raised  above  the  sea,  it  is  said  to  have  reached  the  base- 
level  of  erosion,  or  to  be  base-levelled. 

The  atmospheric  agencies  may  be  conveniently  divided  into 
(i)  rain,  (2)  frost,  (3)  changes  of  temperature,  (4)  wind. 

i.   RAIN 

Perfectly  pure  water,  containing  neither  gases  nor  solids  in  solu- 
tion, would  have  very  little  disintegrating  effect  upon  most  rocks, 
but  pure  water  does  not  occur  in  nature.  The  raindrops,  gener- 
ated by  the  condensation  of  the  watery  vapour  of  the  atmosphere, 
absorb  certain  gases,  which  add  very  materially  to  the  dissolving 
powers  of  the  water.  Of  these  gases  the  most  important  are 
oxygen  (O)  and  carbon  dioxide  (CO.,),  and  all  rain-water  con- 
tains them. 

As  rain-water  percolates  through  the  soil,  it  acquires  additional 
destructive  powers  by  dissolving  the  acids  formed  by  the  decompo- 
sition of  vegetable  matter,  which  are  grouped  together  under  the 
name  of  humous  acids.  These  acids  have  the  power  of  decompos- 
ing or  dissolving  many  minerals  which  resist  the  action  of  ordinary 
rain,  and  thus  it  is  that  many  rocks  which,  when  exposed  to  the 
action  of  the  atmosphere  only,  waste  very  slowly,  disintegrate  with 
comparative  rapidity  underground.  One  of  the  first  and  simplest 
effects  of  atmospheric  moisture  consists  in  the  oxidation  and  hyd ra- 
tion of  the  minerals  exposed  to  it.  Hydration,  or  the  taking  up 
of  water  into  chemical  union,  is  an  important  agency  of  decay. 
It  is  accompanied  by  an  increase  of  bulk,  and  hence  in  the  lower 
layers  of  a  rock-mass  greatly  augments  the  pressure.  In  the 
District  of  Columbia  "granite  rocks  have  been  shown  to  have 
become  disintegrated  for  a  depth  of  many  feet,  with  loss  of  but 
some  13.46  per  cent  of  their  chemical  constituents.  .  .  .  Natural 
joint  blocks  brought  up  from  shafts  were,  on  casual  inspection, 


74  THE  ATMOSPHERE  — RAIN 

sound  and  fresh.  It  was  noted,  however,  that  on  exposure  to  the 
atmosphere,  such  not  infrequently  shortly  fell  away  to  the  condi- 
tion of  sand"  (Merrill). 

Nearly  all  rocks  contain  materials  which  are  more  or  less  solu- 
ble or  destructible  in  rain-water,  though  usually  these  soluble  ma- 
terials make  up  but  a  small  proportion  of  the  whole.  When  they 
are  removed,  the  rock  crumbles  into  a  friable  mass  which,  on 
complete  disintegration,  forms  soil.  Frozen  soil,  though  with  only 
a  small  quantity  of  ice  in  it,  is  as  hard  as  many  rocks,  the  ice  act- 
ing as  a  cement  and  firmly  binding  the  granules  together.  When 
the  cementing  ice  loses  its  coherence  by  melting,  the  rock-like 
mass  becomes  friable.  This  illustrates  the  effect  of  removing  a 
small  quantity  of  soluble  material  from  a  hard  rock.  Except  vege- 
table moulds,  all  soils  are  derived  from  the  disintegration  of  rocks. 

The  materials  out  of  which  rocks  are  made  vary  so  much  that 
the  chemical  processes  which  destroy  them  must  be  correspond- 
ingly different.  Our  survey  of  the  disintegrating  agencies  will 
naturally  begin  with  those  rocks  which  have  once  been  melted, 
or  the  igneous  rocks,  because,  as  we  believe,  the  first  solid  crust 
of  the  globe  was  formed  of  such  rocks,  and  from  them  started  the 
cycles  of  change  and  circulation  of  matter  which  are  still  going  on. 

The  great  majority  of  the  igneous  rocks  are  made  up  of  crystals 
of  some  variety  of  felspar  (see  p.  16)  associated  with  other  min- 
erals, such  as  hornblende  (p.  20),  mica  (p.  18),  quartz  (p.  15),  etc. 
As  an  example,  we  may  take  granite,  which  is  composed  of  ortho- 
clase  felspar,  quartz  (SiO2),  and  mica  or  hornblende,  both  of  which 
are  complex  silicates  of  iron,  lime,  etc.  Rain-water  falling  upon 
an  exposed  mass  of  granite,  or  reaching  it  by  underground  perco- 
lation, slowly  decomposes  the  orthoclase,  either  removing  the 
potash  in  the  form  of  a  soluble  silicate,  or  converting  the  silicate 
into  carbonate.  The  silicate  of  alumina,  which  is  left  behind, 
absorbs  water  and  forms  clay  or  kaolin  (A12O3,  2  SiO2,  2  H2O). 
The  quartz  is  unaffected  by  the  rain-water,  being  insoluble  and 
a  very  simple  and  stable  compound,  ^which  is  not  decomposed 
into  simpler  substances  by  ordinary  natural  agents.  The  mica 
is  very  slowly  disintegrated. 


\  0   -  )  '± 
WEATHERING   OF   ROCKS  75 

The  result  of  the  decomposition  of  granite,  then,  is  the  forma- 
tion of  a  mass  of  clay,  through  which  are  disseminated  the  un- 
changed grains  of  quartz  and  mica.  In  the  other  igneous  rocks 
the  mode  of  disintegration  is  essentially  the  same  :  the  complex 
silicates  are  decomposed  into  simpler  compounds,  clay  being  the 
principal  derivative  of  the  felspars,  while  the  quartz,  if  present,  is 
broken  up  into  fragments  and  forms  sand.  Even  when  an  igneous 
rock  is  yet  firm  and  hard  and,  examined  by  the  naked  eye,  appears 
to  be  quite  unchanged,  the  microscope  often  reveals  important 
chemical  changes,  which  are  the  first  steps  of  decay. 

The  circulation  of  the  material  of  rocks  is  continuous,  and  rocks 
which  are  themselves  composed  of  substances  derived  from  the 
decay  of  older  rocks  are  attacked  in  their  turn  and  yield  material 
for  new  formations.  These  derivative  rocks,  such  as  sandstones, 
slates,  and  limestones,  are  affected  in  characteristic  ways  by  the 
rain. 

Sandstones  are  composed  of  grains  of  sand  (quartz,  SiO2)  ce- 
mented together ;  the  cementing  substance  may  be  silica  itself, 
some  compound  of  iron,  such  as  Fe2O3,  or  carbonate  of  lime 
(CaCO3),  and  the  dissolving  away  of  the  cement  causes  the  rock 
to  crumble  into  sand.  In  a  sandstone  with  siliceous  cement  the 
action  is  excessively  slow,  atmospheric  waters  having  very  little 
effect  upon  silica,  but  underground  the  humous  acids  are  believed  to 
dissolve  it  slightly.  Ferric  oxide  (Fe2O3)  is  likewise  unchanged 
by  rain-water,  but  beneath  the  soil  decomposing  organic  substances 
deoxidize  it  into  FeO,  which,  taking  up  CO2,  forms  the  soluble 
carbonate  of  iron  (FeCO3).  The  uppermost  layers  of  red  sand- 
stone are  often  thus  completely  disintegrated  into  loose  sand, 
bleached  by  the  removal  of  the  iron  which  gave  it  its  colour. 
Carbonate  of  lime  is  very  soluble  in  water  containing  carbon  diox- 
ide, as  all  rain-water  does,  and  in  sandstones  with  calcareous 
cement,  disintegration  is  rapid.  In  sandstones  and  slates  it  is  the 
cementing  substance  which  is  removed,  leaving  the  grains  of  sand 
or  particles  of  clay  unchanged,  and  the  limestones  are  simply  dis- 
solved. This  is  because  the  materials  of  these  rocks  were,  for  the 
most  part,  originally  derived  from  the  decomposition  of  the  igneous 


76  THE  ATMOSPHERE— RAIN 

rocks,  and  the  minerals  which  compose  them  are  already  of  a 
very  simple  and  stable  character. 

The  sandstones  are  largely  employed  for  building  materials,  and 
their  value  and  permanence  for  such  purposes  depend  principally 
upon  the  character  of  the  cementing  substances  in  them.  For 
this  reason,  the  siliceous  and  ferruginous  sandstones  are  the  most 
durable,  those  with  calcareous  cements  usually  yielding  with  com- 
parative rapidity  to  the  attacks  of  the  weather. 

Slates  and  shales,  by  removal  of  their  soluble  constituents, 
crumble  down  into  clay. 

Limestones  are  among  the  few  rocks  which  are  chiefly  or  entirely 
made  up  of  soluble  material,  the  carbonate  of  lime  (CaCO3). 
This  is  attacked  by  the  rain-water,  dissolved  and  carried  away  in 
solution,  while  the  insoluble  impurities  contained  in  the  rock 
remain  to  form  soil.  The  proportion  of  such  impurities  varies 
greatly  in  different  limestones,  and  hence  the  residual  soil  will 
vary,  but  it  is  generally  a  clay,  since  that  is  much  the  commonest 
of  the  impurities  in  limestone.  Sand  also  occurs  in  limestones, 
either  with  or  without  clay.  When  the  sand  forms  a  coherent 
mass,  out  of  which  the  calcareous  material  has  been  dissolved,  it 
is  called  rotten-stone. 

The  gradual  formation  of  soil  by  the  disintegration  of  rock  may 
be  easily  observed  in  excavations,  even  shallow  ones,  such  as  cel- 
lars, wells,  railroad  cuttings,  and  the  like.  At  the  surface  is  the 
true  soil,  which  is  usually  dark  coloured,  due  partly  to  the  admixt- 
ure of  vegetable  mould,  partly  to  the  complete  oxidation  and  hy- 
dration  of  its  minerals.  Next  follows  the  subsoil,  which,  owing  to 
the  absence  of  vegetable  matter  and  the  less  complete  oxidation 
and  hydration,  is  of  a  lighter  colour.  The  subsoil  is  frequently 
divided  into  distinct  layers,  and  often  contains, unaltered  masses 
of  the  parent  rock,  which  have  resisted  decomposition,  while  the 
surrounding  parts  have  become  entirely  disintegrated.  By  im- 
perceptible gradations  the  subsoil  shades  into  what  looks  like 
unaltered  rock,  but  is  friable  and  crumbles  in  the  fingers ;  this  is 
rotten  rock.  From  this  to  the  firm,  unchanged  rock,  the  passage 
is  equally  gradual. 


FORMATION   OF   SOIL 


77 


In  the  northern  portions  of  the  United  States  the  soil  is,  in  most 
localities,  of  only  moderate  depths,  because  at  a  late  period  (geo- 
logically speaking)  this  region  was  covered  with  a  great  ice-sheet, 
which  swept  away  much  of  the 
accumulations  of  ancient  rock- 
decay.  In  the  parts  of  the  coun- 
try where  the  ice-sheet  did  not 
come,  the  soil  is  much  deeper, 
and  in  tropical  lands  it  attains 
remarkable  depths.  In  our  South- 
ern States  the  felspathic  rocks  are 
often  found  thoroughly  disinte- 
grated to  depths  of  50  or  100 
feet,  while  in  Brazil  the  soil  is 
often  200  to  300  feet  deep. 

The  mechanical  effect  of  rain 
is  less  extensive,  perhaps,  than  its 
chemical  work  of  disintegration, 
but  is  very  important,  neverthe- 
less. Under  ordinary  conditions, 
this  mechanical  work  consists  in 
the  washing  of  soil  from  higher 
to  lower  levels.  How  consider- 
able is  the  movement  of  soil  that 
has  thus  been  brought  about,  may 
be  imagined  when  one  sees,  after 
a  heavy  rain,  the  rain-rills  which 
run  over  the  slopes,  muddy  and 

charged  with  sediments,  and  how  turbid  the  streams  become  with 
the  soil  which  the  rain  washes  into  them.  Bare  soil  is  rapidly 
torn  up  and  washed  away  by  the  action  of  rain,  but  a  covering  of 
vegetation,  and  especially  of  the  elastic  and  matted  stems  and 
roots  of  grasses,  much  retards  the  action. 

Other  things  being  equal,  the  rapidity  with  which  the  rain 
sweeps  away  the  soil  depends  upon  the  steepness  of  the  slope 
upon  which  the  soil  is  formed ;  for  gravity  largely  determines 


FIG.  22. —  Excavation  displaying 
the  transition  from  rock  below  to  soil 
above. 


78  THE  ATMOSPHERE  — RAIN 

these  movements.  On  vertical  cliffs  and  steep  hillsides  it  is 
quickly  removed,  and  in  such  places  it  is  thin  or  quite  lacking, 
while  in  the  valleys  it  often  accumulates  to  great  depths.  Even 
on  gentle  slopes  and  almost  level  stretches  the  rains  slowly  wash  it 
downward,  and  eventually  into  the  streams  which  carry  it  to  the 
sea.  The  soil  is  thus  not  stationary,  but  under  the  influence  of 
the  rains  and  streams  is  slowly  but  steadily  travelling  seaward. 
Disregarding  the  alluvial  deposits  made  by  rivers,  and  soils  accu- 
mulated by  the  action  of  ice  or  wind,  the  soil  of  any  district  is 
thus  a  residual  product,  and  its  quantity  represents  the  surplus  of 
chemical  disintegration  over  mechanical  removal. 

The  mechanical  action  of  rain  is  greatly  increased  by  extreme 
violence  and  volume  of  precipitation ;  a  single  "  cloud-burst "  will 
do  far  more  damage  than  the  same  quantity  of  rain  falling  in 
gentle  showers.  Those  who  know  only  the  temperate  regions  can 
form  but  imperfect  conceptions  of  the  violence  of  tropical  rains. 
On  the  southern  foot-hills  of  the  Himalayas,  for  example,  the  rain- 
fall is  exceedingly  great  (in  some  localities  as  much  as  500  inches 
per  annum),  and  almost  all  of  it  is  precipitated  in  six  months  of  the 
year ;  especially  remarkable  is  the  quantity  which  often  falls  in  a 
single  day.  "  The  channel  of  every  torrent  and  stream  is  swollen 
at  this  season,  and  much  sandstone  and  other  rocks  are  reduced 
to  sand  and  gravel  by  the  flooded  streams.  So  great  is  the  super- 
ficial waste,  that  what  would  otherwise  be  a  rich  and  luxuriantly 
wooded  region  is  converted  into  a  wild  and  barren  moorland" 
(Lyell). 

The  action  of  rain  is  thus  by  no  means  uniform,  the  results  de- 
pending upon  so  many  and  such  varying  factors,  that  we  may  find 
marked  differences  in  closely  adjoining  regions,  and  even  in  one 
and  the  same  mass  of  rock.  One  of  the  most  remarkable  monu- 
ments of  rain-erosion  is  exhibited  by  the  curious  districts  in  the 
far  western  states  known  as  the  "bad  lands"  which  cover  many 
thousands  of  square  miles  in  the  Dakotas,  Nebraska,  Wyoming, 
Utah,  etc.  The  bad-land  rocks  are  mostly  rather  soft  sandstones 
and  clays,  with  prevailingly  calcareous  cements,  and  formed  in 
nearly  horizontal  beds  or  layers.  The  rainfall  is  light,  though 


BAD   LANDS 


79 


torrential  showers  sometimes  occur ;  but  the  absence  of  vegeta- 
tion is  favourable  to  its  efficiency,  and  the  present  aridity  of  the 
climate  is  not  of  very  long  standing,  from  a  geological  point  of 
view.  The  chemical  action  of  the  rain  has  disintegrated  the 
rocks  by  dissolving  out  the  calcareous  cement,  and  then  the  debris 
so  formed  has  been  mechanically  washed  away. 

At  the  present  time  the  action  of  the  rain  is  very  slow,  because 
the  debris  which  covers  the  sides  of  the  cliffs  and  slopes  is  almost 


FIG.  23.  — Bad  lands  of  South  Dakota.     (Photograph  by  Williston.) 

impervious  to  water,  and  holes  left  by  the  excavation  of  fossil 
skeletons  often  remain  visible  for  many  years ;  but  where  the 
bare  rock  is  exposed,  the  disintegration  often  proceeds  with  ex- 
traordinary rapidity,  and  a  single  shower  will  produce  notable 
effects.  The  different  layers  of  rock  resist  decay  differently,  and 
even  in  the  same  bed  some  parts  are  much  more  durable  than 
others.  This  differential  weathering  has  resulted  in  that  remarka- 
ble variety  and  grotesqueness  of  form,  resembling  the  ruins  of 
gigantic  towers  and  castles,  for  which  the  bad-land  scenery  is 
famous.  The  sculpture  of  the  rain  produces  this  variety  in  ac- 


80  THE  ATMOSPHERE  — FROST 

cordance  with  the  arrangement  of  the  more  and  less  durable 
layers.  When  the  harder  beds  are  at  the  top,  flat-topped  tables, 
or  mesas,  with  steep  sides,  are  carved  out ;  when  this  hard  bed  is 
removed,  or  was  not  originally  present,  rounded  and  dome-shaped 
hills  and  high,  narrow,  and  precipitous  buttes  -result,  with  the  more 
durable  layers  cropping  out  on  the  sides  as  projecting  ledges. 
Isolated  hard  patches,  by  protecting  the  softer  beds  beneath  them, 
gradually  cause  the  formation  of  pillars,  as  the  unprotected  por- 
tions are  cut  away,  and  these  pillars  may  be  observed  in  all  stages 


FlG.  24. —  Bad-land  peak,  South   Dakota.      The  horizontal  stratification  is  very 
plainly  marked. 

of  their  formation.  Monument  Park,  in  Colorado,  is  especially 
noted  for  this  feature.  Eventually  the  hard  cap  of  the  pillar 
becomes  undermined  and  falls,  and  then  the  shaft  is  speedily 
removed. 

2.    FROST 

The  term  frost,  in  this  connection,  is  restricted  to  the  freezing 
of  water.  Water  is  one  of  the  comparatively  few  substances  which 
expand  considerably  on  passing  from  the  liquid  to  the  solid  state. 
This  expansion,  which  amounts  to  about  one-eleventh  of  the 


DESTRUCTION   OF   ROCK   BY   FROST 


8l 


original  bulk  of  the  water,  takes   place  with  irresistible  power, 
bursting  thick  iron  vessels  like  egg-shells. 

Excepting  loose,  incoherent  masses,  like  sand  and  gravel,  no 
rocks  are  formed  of  continuous  sheets  of  material,  but  are  rather 
to  be  considered  as  masses  of  blocks,  divided  by  ti\s  joints.  (See 
pp.  48  and  200.)  In  addition  to  these  visible  clefts,  the  blocks 
are  traversed  by  minute  crevices,  rifts,  and  pores,  all  of  which 
openings  take  up  and  retain  quantities  of  water,  as  may  readily  be 


FlG.  25.  —  Cliff  and  talus  slope,  Delaware  Water  Gap,  Pa. 

seen  by  examining  freshly  quarried  stone.  When  exposed  to  a 
low  temperature,  the  water  freezes  and  forces  out  the  large  blocks 
and  shatters  them  into  pieces  of  smaller  and  smaller  size.  The 
fragments  thus  formed  are  called  talus,  and  great  accumulations  of 
such  blocks  are  found  at  the  foot  of  cliffs  in  all  regions  where  the 
winters  are  at  all  severe.  Talus  accumulations  are  also  formed  by 
other  agencies,  as  will  be  seen  in  the  sequel.  Alternate  freezings 
and  thawings  not  only  break  up  rocks,  but  cause  the  broken 
fragments  and  soil  to  work  their  way  down  slopes.  Each  freezing 


82  THE  ATMOSPHERE  — FROST 

causes  the  fragments  to  rise  slightly  at  right  angles  to  the  inclined 
surface,  and  each  thawing  produces  a  reverse  movement ;  hence 
the  slow  creep  down  the  slope. 

The  action  of  frost  is,  of  course,  practically  absent  in  the  low- 
lands of  the  tropics,  but  in  high  mountains  and  in  all  countries 


FIG.  26.  —  Shales  "  creeping"  under  the  action  of  frost.     (U.  S.  G.  S.) 

which  have  cold  winters,  frost  is  an  agent  of  great  importance  in 
the  mechanical  shattering  of  rocks  and  slow  destruction  of  cliffs. 
The  hardest  rocks  are  shivered  into  fragments  and,  dislodged  from 
their  places,  the  fragments  roll  down  the  mountain  side  till  they 
come  to  rest,  perhaps  thousands  of  feet  below.  Immense  accumu- 
lations of  frost-made  talus  are  to  be  found  in  such  places  as  the 
foot  of  the  Palisades  of  the  Hudson;  the  abrupt  southern  slope  of 


ACTION   OF   FROST  83 

the  Delaware  Water  Gap,  and  wherever  cliffs  or  peaks  of  naked 
rock  are  exposed  to  severe  cold.  Many  mountain  passes  are  so 
bombarded  by  falling  stones  as  to  be  extremely  dangerous ;  in  the 
Sierra  Nevada  of  California  talus  slopes  as  much  as  4000  feet  high 
are  reported,  all  tire  work  of  frost.  At  Sherman,  where  the  Union 
Pacific  railroad  crosses  the  "  continental  divide,"  the  ground  is 
covered  for  miles  with  small,  angular  fragments  of  granite  broken 
up  by  the  frost. 

In  the  polar  regions  frost  is  probably  the  most  important  of  the 
disintegrating  agents.  In  Spitzbergen  Beechy  found  that  in  sum- 
mer the  mountain  slopes  absorb  quantities  of  water,  which  freezes 
in  winter  with  very  destructive  effect.  "  Masses  of  rock  were,  in 
consequence,  repeatedly  detached  from  the  hills,  accompanied  by 
a  loud  report,  and  falling  from  a  great  height,  were  shattered  to 
fragments  at  the  base  of  the  mountain,  there  to  undergo  more 
rapid  disintegration."  Similar  phenomena  are  reported  from  the 
Aleutian  Islands  of  Alaska. 

The  action  of  frost  is,  in  itself,  purely  mechanical,  no  chemical 
change  is  occasioned  by  it,  and  the  smallest  fragments  into  which  a 
block  may  be  riven  are  sharp  and  angular,  and  the  minerals  have 
unaltered  and  shining  faces.  But,  on  the  other  hand,  frost  pre- 
pares the  way  for  the  more  rapid  action  of  rain  and  percolating 
waters.  The  effects  of  these  agents  are  produced  upon  the  surface 
of  the  rocks  and  the  walls  of  the  crevices  which  run  through  them. 
By  breaking  up  the  blocks,  the  frost  greatly  increases  the  surface 
and  thus  facilitates  the  work  of  the  rain.  An  example  will  make 
this  clear.  A  cube  of  stone,  measuring  one  foot  each  way,  has  six 
sides,  each  of  144  square  inches,  and  its  total  superficies  is  thus 
144  x  6  =  864  square  inches.  Suppose  this  block  to  be  riven  by 
the  frost  into  pieces  of  one  cubic  inch  each  ;  of  such  small  cubes 
there  will  be  1728,  each  with  six  square  inches  of  surface,  giving 
10,368  square  inches  of  superficies  for  all  the  cubes.  A  breaking 
up  of  the  cubic  foot  into  cubic  inches  thus  multiplies  the  exposed 
surface  by  12. 

Rain  and  frost  are  agents  whose  effects  are  most  important  in 
regions  of  moist  climate  and  abundant  rainfall,  for  both  are  forms 


84 


THE   ATMOSPHERE 


of  the  activity  of  water.  Few  regions  of  the  earth's  surface  are 
altogether  rainless,  but  nearly  all  of  the  continents  have  great 
desert  areas  in  which  atmospheric  precipitation  is  very  light.  It 
might  seem  that  in  such  deserts  the  work  of  rock  disintegration 


FlG.    27.  —  Weathered    and    exfoliating    granite,    Sierra    Nevada,    California. 

(U.  S.  G.  S.) 

must  be  practically  at  a  standstill,  and  that  the  circulation  of  ma- 
terial must  be  so  slow  as  to  be  hardly  distinguishable  from  com- 
plete stagnation.  Even  in  these  regions,  however,  the  rain  accom- 
plishes something,  and  it  is  aided  by  other  agencies  which  in  moist 
climates  play  a  much  more  modest  role ;  these  are  the  changes  of 
temperature  and  the  wind. 


ACTION  OF  WIND  85 

3.   CHANGES  OF  TEMPERATURE 

These  changes,  by  alternately  expanding  and  contracting  the 
rocks,  widen  the  crevices  and  fissures  which  traverse  them.  In 
humid  climates  this  agency  is  a  very  subordinate  one,  and  acts 
chiefly  in  preparing  an  easier  path  for  percolating  waters,  but 
in  dry  regions  it  becomes  much  more  important.  In  the  latter 
case,  the  naked  rocks  are,  during  the  day,  heated  to  a  high 
temperature  by  the  full  blaze  of  the  sun,  and  at  night  the  rapid 
radiation  which  occurs  in  dry  air,  cools  them  very  quickly.  When 
radiation  begins,  the  outer  layers  of  the  rock  chill  rapidly  and 
attempt  to  contract  upon  the  still  heated  and  therefore  expanded 
interior ;  thus  strains  are  set  up  which  the  rock  cannot  resist,  and, 
therefore,  great  pieces  are  split  off.  In  this  fashion  talus  slopes  of 
angular  blocks  form  at  the  foot  of  the  cliffs,  just  as  in  the  case  of 
frost-made  talus,  and  this  work  goes  on  in  all  arid  regions  which 
have  hot  days  and  cool  nights.  The  agency  is  purely  mechanical 
and  effects  no  chemical  change ;  it  is  also  entirely  superficial  and 
is  prevented  by  even  a  thin  covering  of  soil. 

The  work  of  destruction  due  to  changes  of  temperature  goes 
farther  than  merely  splitting  blocks  off  the  faces  of  cliffs,  and  may 
result  in  breaking  up  a  rock  into  minute  fragments.  Compara- 
tively few  rocks  are  made  up  of  a  single  mineral,  and  in  many 
rocks  several  varieties  of  minerals  occur.  Each  of  these  minerals 
will  expand  and  contract,  when  heated  or  chilled,  at  a  slightly 
different  rate  from  the  others,  and  thus  the  particles  are  subjected 
to  stresses  which  will  gradually  loosen  them,  causing  the  rock  to 
disintegrate. 

4.   WIND 

Of  itself  the  wind  is  unable  to  accomplish  in  any  important 
degree  the  disintegration  of  firm  rocks,  but  when  it  can  drift  along 
sand  and  fine  gravel,  it  may  effect  much.  Except  on  sandy  coasts, 
this  agency  is  of  small  importance  in  regions  of  ordinary  rainfall, 
because  in  these  the  soil  is  protected  and  held  together  by  its 
covering  of  vegetation.  In  arid  regions  and  deserts,  on  the  con- 


86  THE  ATMOSPHERE— WIND 

trary,  high  winds  sweep  along  quantities  of  sand  and  fine  gravel, 
which  are  hurled  against  any  obstacle  and  gradually  cut  it  away. 

Very  hard  rocks  yield  but  slowly  to  the  cutting  action  of  wind- 
driven  sand,  and  in  them  the  chief  effect  to  be  observed  is  a 
scratching  and  polishing  of  the  surface.  The  same  principle  is 
employed  in  the  sand-blast,  which  is  a  jet  of  sand,  driven  at  a 
high  velocity  and  used  to  engrave  glass,  polish  granite,  and  do 
other  work  of  the  kind.  Soft  rocks  are  quite  rapidly  abraded  and 
cut  down  by  the  drifting  sand,  and  go  to  increase  the  mass  of 
cutting  material.  The  softer  parts  are  cut  away  first,  leaving  the 
harder  layers,  streaks,  or  patches  standing  in  relief.  In  this  way 
very  fantastic  forms  of  rocks  are  frequently  shaped  out :  pot-holes 
and  caverns  are  excavated  by  the  eddying  drift,  and  archways 
cut  through  projecting  masses. 

As  the  wind  does  not  lift  the  harder  and  heavier  particles  to 
any  great  height,  the  principal  effect  is  produced  near  the  level 
of  the  ground,  and  thus  masses  of  rock  are  gradually  undermined 
and  fall  in  ruins,  which  in  their  turn  are  slowly  abraded.  Isolated 
blocks  are  sometimes  so  symmetrically  cut  away  on  the  under  side, 
that  they  come  to  rest  upon  a  very  small  area  and  form  rocking- 
stones,  which,  in  spite  of  their  size  and  weight,  may  be  swung  by 
the  hand. 

The  fine  particles  abraded  from  the  rocks  by  drifting  sand  have* 
undergone  no  chemical  change,  the  process  being  entirely  me- 
chanical. 

The  abrading  effects  of  wind-driven  sand  may  be  observed  in 
any  desert  region  where  naked  rocks  are  exposed,  as  for  example 
in  the  arid  parts  of  Utah  and  Arizona.  One  very  characteristic 
effect  of  this  natural  sand-blast  is  found  in  the  appearance  of  the 
pebbles  shaped  by  it.  Pebbles  of  very  hard  and  homogeneous 
materials,  such  as  quartz  or  chalcedony,  are  highly  polished. 
Those  made  from  igneous  rocks  have  the  softer  minerals  worn 
away,  leaving  the  harder  to  stand  in  relief  in  curious  patterns, 
while  limestone  is  carved  into  beautiful  arabesques. 

We  have  seen  that  the  rain  is  slowly  shifting  the  soil  seaward, 
and  in  dry  countries  the  wind  acts  in  similar  fashion.  Strong 


ROCK   DESTRUCTION   IN  ARID   REGIONS  87 

winds,  blowing  steadily  in  one  direction,  carry  great  quantities  of 
dust  and  fine  sand  with  them,  sometimes  directly  into  the  sea  or 
other  bodies  of  water,  sometimes  into  rivers,  or  again  to  moister 
regions,  where  it  comes  under  the  influence  of  the  rain. 

Slowly  as  they  work,  the  wind  and  temperature  changes  prevent 
any  complete  stagnation  in  the  circulation  of  material,  and  thanks 
to  them,  the  processes  of  disintegration  of  rock  and  transporta- 
tion of  soil  are  kept  up  even  in  the  dryest  deserts. 


CHAPTER    V 

DESTRUCTIVE   PROCESSES  —  RUNNING   WATER 

THE  source  of  all  running  water,  whether  surface  or  under- 
ground, is  atmospheric  precipitation  in  the  form  of  rain  or  snow. 
All  springs  and  streams  are  merely  rain  (or  snow)  water  collected 
together  and  fed  from  reservoirs.  Of  the  rain-water  which  falls 
upon  the  land,  about  one  third  is  evaporated,  a  second  third  flows 
over  the  surface  to  the  nearest  water-course,  and  the  remainder 
sinks  into  the  soil  to  a  greater  or  less  depth,  and  though  part  of 
it  again  comes  to  the  surface  in  springs,  yet  a  great  portion  must 
reach  the  sea  by  subterranean  channels. 

i.   UNDERGROUND  WATERS 

The  flow  of  underground  waters,  as  well  as  that  of  surface 
waters,  is  determined  by  gravity,  but  surface  topography  has  no 
effect  upon  it,  and  often  the  superficial  and  subterranean  flows  are, 
for  considerable  distances,  in  exactly  opposite  directions.  Un- 
derground drainage  is  determined  by  the  inclination  of  the  rocks, 
the  alternation  of  porous  and  impervious  beds,  the  number  and 
character  of  their  joints  and  fissures,  for  the  flow  is  ordinarily 
through  these  crevices,  except  in  very  porous  materials,  such  as 
loose  sand,  which  allow  of  a  flow  through  their  substance.  In 
soluble  rocks  the  water  may  dissolve  out  its  own  channels. 

These  facts  of  the  underground  movement  of  water  are  of  great 
practical  importance  in  all  questions  of  drainage  and  water  sup- 
ply. Serious  evils  have  followed  from  the  careless  assumption 
that  subterranean  drainage  would  be  in  the  same  direction  as  that 
on  the  surface  of  the  ground.  The  accompanying  diagram  shows 
an  arrangement  of  cess-pool  and  well,  which  was  planned  on  the 


SOLUTION   BY   UNDERGROUND   WATERS  89 

assumption  that  because  the  former  was  farther  down  the  hill,  it 
could  not  contaminate  the  latter.  But  the  inclination  of  the  rocks 
is  such  that  the  cess-pool  would  drain  as  directly  into  the  well  as 
though  a  pipe  connected  them.  Underground  waters  perform  the 
work  of  rock  disintegration,  both  chemically  and  mechanically, 


FIG.  28.  —  Diagram  illustrating  how  surface  and  underground  drainage  may  be  in 
opposite  directions. 

but  as  the  movement  of  such  waters  is  usually  extremely  slow,  the 
mechanical  work  is  of  very  subordinate  importance. 

In  considering  the  effects  of  the  rain  we  learned  that  its  chem- 
ical efficiency  is  much  increased  by  the  humous  acids  which  it 
takes  up  on  passing  through  the  soil.  The  water,  making  its  way 
downward  through  the  rocks,  by  means  of  the  joints  and  bedding 
planes,  exerts  its  slowly  dissolving  and  decomposing  effects  upon 
the  walls  of  these  crevices.  Such  water  therefore  always  contains 
more  or  less  mineral  matter  in  solution,  the  nature  and  quantity  of 
which  depend  upon  the  character  of  the  rock  traversed. 

In  passing  through  limestones,  percolating  waters  produce  re- 
markable effects,  owing  to  the  solubility  of  the  rock.  From  the 
surface  sinkholes  and  pipes  are  dissolved  downward,  while  in  the 
mass  of  the  rock  caverns  are  dissolved  out,  often,  as  in  the  Mam- 
moth Cave  of  Kentucky,  many  miles  in  extent  and  with  rivers  of 


9O  RUNNING   WATER 

considerable  size  flowing  in  them.  Indeed,  in  limestone  regions 
the  smaller  streams  are  generally  engulfed  and  flow  for  a  longer 
or  shorter  distance  underground.  When  the  roof  of  a  cavern  is 
no  longer  able  to  support  itself,  it  falls  in  and  exposes  a  ravine. 
Any  portion  of  the  roof  which  remains  standing  will  then  form  a 
bridge,  examples  of  which  are  the  famous  Natural  Bridges  of 
Virginia  and  of  the  Tonto  Basin  in  Arizona. 

Nothing  is  known  as  to  the  limits  of  depth  to  which  the  perco- . 
lating  waters  may  penetrate  the  crust  of  the  earth,  but  so  far  as 
borings  and  deep  mines  have  gone,  water  is  always  found,  and  the 
limit  is  probably  far  below  any  yet  reached  artificially.  As  the 
temperature  of  the  earth  increases  downward,  a  level  must  be 
attained  (probably  at  no  very  great  depth)  where  the  rocks 
become  too  hot  to  allow  any  further  penetration,  and  at  such 
depths  the  great  pressure  of  the  overlying  masses  must  tend  to 
close  up  the  joints  and  crevices  through  which  the  water  descends , 
The  moisture  in  deep-seated  rocks  must  be,  for  the  most  part, 
stationary  or  subject  only  to  very  slow  fluctuations,  for  such  rock's 
are  solid  and  undecomposed.  Even  beds  of  rock  salt,  which, 
would  surely  be  dissolved  away  by  moving  water,  are  found  at 
depths  which  can  be  reached  by  mining  or  boring. 

When  underground  waters  become  highly  heated  by  descending 
to  great  depths  along  channels  which  admit  of  a  return  to  higher 
levels,  or  by  coming  in  contact  with  masses  of  hot  lava,  their 
solvent  efficiency  is  greatly  increased.  Rocks  penetrated  by  such 
thermal  waters  are  often  profoundly  altered  in  their  character 
and  composition.  In  igneous  rocks  so  treated  the  complex 
minerals  which  make  up  these  rocks  are  decomposed  into  simpler 
or  more  stable  compounds.  The  felspars  become  opaque  from 
the  formation  of  kaolin,  or  are  transformed  into  hydrated  mica ; 
minerals  containing  iron  and  magnesia  give  rise  to  chlorites 
(p.  21),  serpentine  (p.  22),  and  the  like,  while  the  lime  com- 
pounds are  converted  into  the  carbonate  and  carried  away  in 
solution.  Some  of  the  minerals  are  altered  in  place,  and  others 
are  deposited  in  the  fissures  and  cavities  of  the  rock.  Thermal 
waters  also  alter  the  character  of  rocks  by  bringing  in  new 


THERMAL   WATERS  91 

material   from   elsewhere.      In    the  Yellowstone    Park   the    great 
lava  mass,  which  has  been  trenched  by  the  Yellowstone  Canon, 


FiG.  29.  —  Natural  Bridge,  Virginia.     (U.  S.  G.  S.) 


92 


RUNNING   WATER 


has  been  profoundly  altered  and  decomposed  by  the  action  of 
the  heated  waters  which  traverse  it. 

Except  in  limestone  caverns,  underground  waters  seldom  flow 
with  sufficient  velocity  to  accomplish  much  in  the  way  of  direct 
mechanical  erosion,  but  indirectly  they  bring  about  mechanical 
changes  of  some  importance.  Slopes  of  earth  or  talus  blocks 
lying  on  hillsides  or  mountains,  saturated  by  long-continued,  heavy 
rains,  may  have  their  weight  so  much  increased  and  their  friction 
so  reduced,  as  to  glide  downward  in  landslips,  which  in  inhabited 
regions  are  sometimes  very  destructive.  Of  this  kind  was  the 
great  landslip  which  occurred  in  the  White  Mountains  (New 
Hampshire)  in  1826. 

Landslips  may  also  occur  when  the  rocks  forming  a  slope  are 
inclined  in  the  same  direction  as  that  slope ;  the  surface  layers 
weighted  with  water,  and  especially  if  underlaid  by  clay-beds, 
which  when  lubricated  with  water  become  very  slippery,  may 
glide  down  the  slope  into  the  valley  below.  Mountain  valleys  in 
all  parts  of  the  world  show  plain  evidence  of  such  landslips,  and 
the  amount  of  rock  thus  displaced  is  sometimes  very  great.  The 
landslip  which  occurred  at  Elm,  Switzerland,  in  1881,  is  esti- 
mated to  have  carried  down  more  than  12,000,000 
cubic  yards  of  rock  for  a  distance  of  2000  feet. 


2.   SPRINGS. 

Springs  are  the  openings  of  un- 
derground streams  upon   the 
surface,  and  could  not  be 
formed  were  the  land 
perfectly   free   from 

FIG.  30.  —  Arrangement  of  strata  which  causes  hillside  .  . 

springs.     The  lower  close-lined  bed  impervious.  irregularities,          for 

gravity  controls   the 

movement  of  underground  waters,  and  the  source  of  a  spring  must 
be  higher  than  its  mouth.  It  must  be  remembered,  however,  that 
a  subterranean  stream  is  often  confined  as  in  a  pipe,  and  that  the 
pressure  to  which  it  is  subjected  may  seem  to  make  it  flow  upward, 


FISSURE   SPRINGS 


93 


as  when  a  spring  rises  from  a  deep  fissure,  or  bursts  out  upon 
the  top  of  a  hill.  But  these  are  not  real  exceptions,  and  here 
also  the  source,  which  may  be  many  miles  distant,  is  above  the 
spring,  and  it  is  this  which  produces  the  necessary  pressure. 

The  commonest  type  of  spring  is  formed  when  a  relatively  im- 
pervious bed  of  rock  (usually  clay  in  some  form)  overlaid  by 
porous  rocks,  crops  out  on  a  hillside.  The  rain-water  descends 
through  the  porous  beds,  saturating  their  lower  layers,  until  its 


FlG.  31. —  Diagram  of  fissure-spring.    The  heavy  line  represents  the  fissure  along 
which  the  water  rises. 


descent  is  arrested  by  the  impervious  bed,  and  then  the  water 
follows  the  upper  surface  of  the  latter.  When,  by  some  irregu- 
larity of  the  ground,  the  impervious  bed  comes  to  the  surface,  the 
water  will  issue  as  a  spring,  or  a  line  of  springs.  (See  Fig.  30.) 

A  second  class  of  springs  are  those  which  rise  through  a  crack 
or  fissure  in  the  rocks.  Inclined  porous  beds,  enclosed  between 
more  impervious  ones,  allow  the  water  to  follow  them  downward, 
until  in  its  lower  course  such  water  is  under  great  pressure,  or 
"  head."  On  reaching  a  fissure  opening  upward,  the  water  will  rise 
through  it  and,  if  under  sufficient  pressure,  will  come  to  the  surface. 


94  RUNNING   WATER 

An  artesian  well  is  a  boring  which  taps  a  subterranean  stream 
or  sheet  of  water,  confined  under  sufficient  pressure  to  rise  to  the 
surface,  or  even  spout  above  it,  like  a  fountain.  Artesian  wells 
form  most  valuable  sources  of  water  supply,  and  it  is  important  to 
understand  the  conditions  under  which  they  may  be  successfully 
bored.  Many  people  have  the  impression  that  a  boring  anywhere, 
if  deep  enough,  will  furnish  artesian  water,  and  much  money  has 
been  wasted  by  making  this  assumption.  The  only  safe  guide  is 
a  careful  examination  of  the  geological  structure  of  the  region, 
and  failure  may  result  even  when  everything  seems  favourable. 

The  structural  requisites  for  successful  borings  are  as  follows  : 
(i)  There  must  be  a  porous  water-bearing  stratum,  usually  sand 
or  sandstone,  enclosed  between  relatively  impervious  beds.  The 
impervious  beds  are  necessary  to  enclose  the  porous  beds  and 
prevent  the  water  from  escaping  either  upward  or  downward, 
shutting  it  in  as  in  a  closed  pipe,  or  underground  siphon.  (2)  At 
some  point,  which  must  be  above  the  mouth  of  the  well,  the 
porous  stratum  must  reach  the  surface,  so  that  it  may  receive  its 
supply  of  rain-water.  If  all  the  points  where  the  porous  bed 
crops  out  be  above  the  mouth  of  the  proposed  well,  the  conditions 
are  particularly  favourable,  because  none  of  the  water  can  escape, 
except  by  very  slow  percolation  through  the  relatively  impervious 
beds.  If  the  water-bearing  stratum  communicates  with  the  surface 
at  a  level  lower  than  the  site  of  the  well,  success  will  depend  upon 
the  ease  with  which  the  water  can  escape  at  the  lower  level.  The 
best  conditions  for  a  flowing  well  are,  therefore,  to  be  found  in 
a  basin  of  folding,  with  the  strata  dipping  toward  the  well  from 
all  directions,  and  the  porous  bed  cropping  out  around  the  edge 
of  the  basin,  at  levels  much  above  the  mouth  of  the  well. 

The  outcropping  edges  of  the  porous  strata  may  be  very  far, 
even  hundreds  of  miles,  from  the  well;  the  water  will  follow  the 
dip  of  the  beds  and  rise  to  the  surface  wherever  it  is  tapped, 
provided  that  there  is  no  easier  path  of  escape.  The  friction  of 
the  slow  creeping  through  the  water-bearing  bed  is  so  great  that 
a  spouting  well  never  throws  the  water  up  to  the  level  of  the 
outcrop. 


DESTRUCTIVE   ACTION   OF   SPRINGS  95 

The  quantity  of  water  which  can  be  annually  drawn  from  an 
artesian  reservoir  cannot  exceed  the  annual  supply  through  rain- 
fall, though,  at  first,  the  stored  water  will  cause  a  more  abundant 
(low.  When  several  wells  are  bored  near  together,  each  new 
well  is  apt  to  diminish  the  flow  from  the  older  ones,  though  the 
total  discharge  from  all  the  wells  increases  until  the  limit  of  supply 
is  reached.  New  wells  at  lower  levels  may  take  all  the  water  and 
leave  none  for  those  at  higher  levels. 

While  the  basin-like  arrangement  of  strata  is  the  most  favour- 
able, it  is  not  necessary.  Along  the  New  Jersey  coast  is  a  great 
thickness  of  alternating  beds  of  <-ands  and  clays,  all  dipping  gently 
toward  the  sea.  Aside  from  levels  of  minor  importance,  no  less 
than  six  separate  porous  layers  afford  abundant  supplies  of  water, 
which  are  now  very  largely  drawn  upon. 

The  depth  necessary  to  obtain  flowing  wells  differs,  of  course, 
with  the  topography  and  structure  of  the  country.  In  North 
Dakota,  for  example,  flowing  wells  have  been  obtained  at  depths 
varying  from  less  than  100  to  more  than  1500  feet.  The  Cre- 
taceous sandstone  which  underlies  immense  areas  of  the  Great 
Plains  region  is  the  principal  source  of  supply. 

In  limestone  districts  ravines  may  intersect  the  course  of  con- 
siderable underground  streams,  which  thus  reach  the  surface  in 
springs  of  unusual  volume.  A  very  striking  and  beautiful  example 
is  the  Giant  Spring  in  the  canon  of  the  upper  Missouri,  near  Great 
Falls,  Montana. 

Springs,  as  such,  do  little  in  the  way  of  rock  disintegration,  but 
they  accomplish  something  by  undermining  the  rocks  at  the  point 
where  they  issue,  and  thus  working  their  way  backward.  This 
process  is  known  as  the  recession  of  spring-heads.  The  under- 
ground streams,  of  which  springs  are  the  outlets,  have  often  ef- 
fected much  in  the  way  of  dissolving  rock-material,  and  hence 
spring-water  always  contains  dissolved  minerals,  principally  the 
carbonates  and  sulphates  of  lime  and  magnesia,  and  the  chlo- 
rides of  magnesium  and  sodium.  In  mineral  springs  the 
quantity  of  dissolved  materials  is  larger  and  perceptible  to  the 
taste. 


96  RUNNING   WATER 

Thermal  Springs  are  those  whose  temperature  is  notably  higher 
than  that  of  ordinary  springs  in  the  same  region,  and  they  range 
from  a  luke-warm  to  a  boiling  state.  This  increase  of  temperature 
maybe  caused  in  either  of  two  ways:  (i)  In  volcanic  regions, 
water  coming  into  contact  with  uncooled  masses  of  lava  is  highly 
heated  and  reaches  the  surface  as  a  hot  spring.  Of  this  class 
are  the  innumerable  thermal  springs  of  the  Yellowstone  Park. 
(2)  Wherever  the  disposition  of  the  rocks  is  such  that  water  may 
descend  to  great  depths  within  the  earth  and  yet  return  to  the 
surface  by  hydrostatic  pressure,  thermal  springs  appear.  These 
conditions  are  found  only  in  regions  where  the  rocks  have  been 
much  folded  and  fractured.  In  this  case  the  temperature  of  the 
water  is  raised  by  the  interior  heat  of  the  earth,  which,  as  we 
have  seen,  increases  with  the  depth.  Springs  of  this  class  occur 
numerously  along  the  Appalachian  Mountains,  and  in  larger  num- 
bers and  of  higher  temperatures  they  accompany  the  various 
ranges  of  the  Rocky  Mountains  and  Sierra  Nevada. 

Geysers  are  thermal  springs  which  periodically  erupt,  throwing 
up  hot  water  in  beautiful  fountains,  accompanied  by  clouds  of 
steam.  Though  of  great  scientific  interest,  geysers  are  not  im- 
portant geological  agents,  because  of  their  rarity,  since  they  occur 
only  in  Iceland,  the  Yellowstone  Park,  and  New  Zealand. 

The  destructive  effects  of  thermal  springs  are  principally  ac- 
complished below  the  surface,  and  have  already  been  considered 
under  the  head  of  underground  waters.  The  high  percentages  of 
dissolved  materials  which  such  springs  usually  contain  are  evidence 
of  the  important  work  of  rock  disintegration  which  they  perform. 

3.   RIVERS 

The  destructive  work  of  rivers,  including  in  that  term  all  surface 
streams,  is  far  less  extensive,  in  the  aggregate,  than  that  of  the 
atmospheric  agencies,  but  because  the  work  of  a  stream  is  concen- 
trated along  its  narrow  course,  it  appears  much  more  striking  and 
impressive. 

The  chemical  disintegration  performed  by  rivers  is  of  no  great 


RIVER   EROSION  97 

amount,  though,  of  course,  some  soluble  materials  are  withdrawn 
from  the  rocks,  over  which  the  waters  flow.  In  limestones  this 
may  be  considerable,  especially  if  the  water  be  charged  with 
organic  acids  from  a  swamp  or  peat-bog. 

The  mechanical  work  of  a  river  is  much  greater  than  the  chemi- 
cal, and  is  dependent  upon  the  velocity  of  the  current,  varying 
directly  as  the  square  of  that  velocity.  The  velocity  of  a  stream 
is  the  rather  complex  resultant  of  several  factors,  the  chief  of  which 
is  gravity ;  the  steeper  the  slope  of  the  bed,  the  swifter  the  flow  of 
the  water.  A  second  factor  is  the  volume  of  water,  the  velocity 
varying  as  the  cube  root  of  the  volume.  That  is  to  say,  if  one  of 
two  streams  which  flow  down  the  same  slope  has  eight  times  as 
much  water  as  the  other,  it  will  flow  twice  as  fast.  Other  factors 
enter  into  the  result,  but  slope  of  bed  and  volume  of  water  are 
much  the  most  important. 

Pure  water  can  do  little  to  abrade  hard  rocks,  though  it  can 
wash  away  sand,  gravel,  and  other  loose  materials.  As  in  the  case 
of  the  wind,  the  stream  merely  supplies  the  power ;  the  implement 
with  which  the  cutting  is  performed  is  the  sand,  pebbles,  and  other 
hard  particles,  which  the  water  sets  in  motion.  These  abrade  the 
rocks  against  which  they  are  cast,  just  as  the  wind-driven  sand 
does,  but  more  effectively,  because  of  the  ceaseless  activity  of  the 
stream,  and  because  many  rocks  are  rendered  softer  and  more 
yielding  by  being  wet.  The  cutting  materials  are  themselves 
abraded  and  worn  finer  and  finer  by  continued  friction  against  the 
rocks  and  against  one  another.  In  the  case  of  complex  minerals 
this  abrasion  is  accompanied  by  more  or  less  chemical  decompo- 
sition, as  has  been  shown  experimentally  by  rotating  crystals  of 
felspar  in  a  drum  half  filled  with  water.  When  the  felspar  was 
ground  down  to  mud,  the  water  showed  the  presence  of  potash 
and  soda  in  solution. 

A  river  which  is  subject  to  sudden  fluctuations  of  volume,  being 
now  a  rushing  torrent  and  again  almost  dry,  is  a  much  more  effi- 
cient agent,  both  of  erosion  and  of  transportation,  than  is  one 
which  carries  nearly  the  same  quantity  of  water  at  all  times,  or 
which  fluctuates  only  slowly. 


98  RUNNING   WATER 

Since  the  velocity  of  a  stream  is  so  largely  dependent  upon 
gravity,  it  is  obvious  that  the  deeper  a  stream  cuts  its  channel,  the 
less  steep  does  its  slope  become,  and  that  so  long  as  the  region  is 
neither  upheaved  nor  depressed,  the  river  performs  its  vertical 
erosion  at  a  constantly  decreasing  rate.  Unless,  therefore,  the 
work  is  done  under  very  exceptional  conditions,  as  in  the  case  of 
the  Niagara,  we  cannot  reason  from  the  present  rate  of  excavation 
to  the  length  of  time  involved  in  cutting  out  a  given  gorge. 

Unless  the  region  through  which  a  river  flows  is  upheaved,  and 
thus,  by  increasing  the  fall,  renewed  power  is  given  to  the  stream, 
a  stage  must  sooner  or  later  be  reached  when  the  vertical  cutting 
of  the  stream  must  cease.  This  stage  is  called  the  base-level  of 
erosion,  or  regimen  of  the  river,  and  it  approximates  a  parabolic 
curve,  rising  toward  the  head  of  the  stream.  Elevation  of  the 
country  will  start  the  work  afresh,  until  a  new  base-level  is  reached, 
while  depression  will  have  a  contrary  effect  and  may  put  a  stop  to 
vertical  erosion  where  it  was  in  active  progress  before.  When 
the  base-level  is  reached,  the  river  cuts  laterally,  undermining  its 
banks  and  working  like  a  horizontal  tool  upon  the  country-side. 

So  long  as  the  slope  of  the  bed  is  steep,  the  river  runs  swiftly 
and  with  a  comparatively  straight  course ;  when  lower  slopes  are 
reached,  the  stream  begins  to  meander,  and  shift  its  channel 
about,  undermining  its  banks,  cutting  them  away  in  one  place  and 
building  them  up  in  another,  and  forming  a  wide  plain. 

Having  learned  the  general  character  of  river  erosion,  we  may 
illustrate  it  with  a  few  concrete  examples. 

i.  A  particularly  interesting  case  is  that  of  the  little  river 
Simeto  in  Sicily,  since  the  history  of  its  gorge  is  so  well  known. 
In  1 603  a  great  lava  flood  from  ^Etna  was  poured  out  across  the 
course  of  the  stream,  and,  when  cold,  solidified  into  a  barrier  of 
the  hardest  rock.  When  Sir  Charles  Lyell  visited  the  spot  in 
1828,  he  found  that  in  a  little  more  than  two  centuries  the  stream 
had  cut  a  gorge  through  this  barrier  of  40  to  50  feet  deep,  and 
varying  in  width  from  50  to  several  hundred  feet.  The  lava  which 
had  thus  been  trenched  is  not  porous  or  slaggy,  but  homogeneous 
and  dense. 


EXAMPLES   OF   RIVER   ACTION 


99 


2.  In  the  northern  parts  of  the  United  States  the  great  ice- 
sheet,  which  in  late  geological  times  covered  the  country,  brought 
down  with  it  vast  quantities  of  drift,  that  filled  up  the  channels 


FIG.  32.  —  The  Au  Sable  Chasm,  N.Y.    (Copyright  by  S.  R.  Stoddard,  Glens  Falls,  N.Y.) 


100  RUNNING   WATER 

of  many  streams  and  quite  revolutionized  the  drainage  of  certain 
districts.  Since  that  time  the  displaced  streams  have  cut  out 
new  channels  for  themselves,  often  through  hard  rocks,  and 
many  now  flow  in  quite  deep  gorges,  with  nearly  vertical  walls. 
Au  Sable  Chasm,  New  York,  is  an  example  of  these  geologically 
modern  river  gorges,  the  atmosphere  not  having  had  time  to 
widen  it. 

3.  The  Niagara  is  an  exceptional  case,  the  gorge  being  cut, 
not  only  by  the  direct  abrasion  of  the  running  water,  but  also  by 
the  action  of  the  spray  and  frost  at  the  falls.     In  the  ravine  the 
upper  rock  is  a  hard,  massive  limestone,  which  is  underlaid  by  a 
soft  clay-shale.     The   latter  is  continually   disintegrated   by   the 
spray  of  the  cataract  and  by  the  severe  winter  frosts,  undermin- 
ing the  limestone,  which,  when  no  longer  able  to  bear  its  own 
weight,  breaks  off  in  tabular  masses.     Thus  the  falls  are  steadily 
receding,  leaving  behind  them  a  gorge,  which  is  deepened  by  the 
river. 

4.  The  most  remarkable  known  examples  of  river  erosion  are 
the  canons  of  the  Colorado.     The  Grand  Canon  is  over  200  miles 
long  and  from  4000  to  6500  feet  deep,  with  precipitous  walls.     It 
is  extremely  probable  that  the  river  has  been  rendered  able  to  cut 
to  such  profound   depths   by  the  gradual  uplifting  of  the  whole 
region,  which  is  now  a  lofty  plateau,  in  places  more  than  8000 
feet  above  the  sea.     The  erosive  power  of  the  river  has  thus  been 
continually  renewed  and  a  more  or  less  uniform  rate  of  excavation 
secured.     (See  Frontispiece.) 

Transportation  by  Rivers.  — The  main  importance  of  rivers  as 
geological  agents  is  not  their  work  of  erosion,  but  lies  rather  in 
what  they  accomplish  as  carriers  of  the  results  of  their  own  destruc- 
tive activity  and  that  of  the  atmosphere,  comprising  both  the 
materials  which  are  mechanically  swept  along  in  suspension  and 
those  which  are  carried  in  solution. 

Materials  in  Suspension.  —  The  transporting  power  of  running 
water  is  dependent  upon  the  velocity  of  the  current,  and  both 
mathematical  and  experimental  treatment  of  the  problem  brings 
out  the  surprising  result  that  the  transporting  power  varies  directly 


DISSOLVED   MATERIALS  IOI 

as  the  sixth  power  of  the  velocity.  If  the  rapidity  of  a  stream  be 
doubled,  it  can  carry  64  times  as  much  as  before.  The  destruc- 
tiveness  of  sudden  and  violent  floods  is  thus  explained.  In  the 
terrible  flood  which  overwhelmed  Johnstown,  Pennsylvania,  in 
1889,  great  locomotives  and  massive  iron  bridges  were  swept  off, 
it  is  hardly  an  exaggeration  to  say,  like  straws,  and  huge  boulders 
carried  along  like  pebbles. 

It  obviously  follows  from  the  relation  obtaining  between  velocity 
and  transporting  power,  that  a  slight  increase  in  the  rapidity  of  a 
stream  will  largely  augment  the  load  which  it  carries,  provided  the 
stream  obtains  as  much  material  as  it  can  carry,  while  a  slight 
reduction  of  velocity  will  cause  the  deposition  of  a  large  part  of 
that  load.  The  buoyancy  of  water  adds,  in  an  important  degree, 
to  its  ability  to  sweep  along  sediment,  because  when  any  substance 
is  immersed  in  water,  it  loses  weight  to  an  amount  equal  to  the 
weight  of  an  equal  bulk  of  water.  The  specific  gravity  of  most 
rocks  is  from  two  and  one-half  to  three,  so  that  when  immersed 
they  lose  from  one-third  to  two-fifths  of  their  weight  in  air.  The 
shape  of  the  fragments  is  likewise  a  factor  in  determining  the 
velocity  requisite  to  move  them  ;  the  larger  the  surface  of  the 
fragment  in  proportion  to  its  weight,  the  more  easily  it  is  carried 
in  suspension.  Thus  flat  grains  or  scales  are  carried  farther  than 
round  ones  ;  while,  on  the  other  hand,  rounded  fragments  are  more 
easily  rolled  along  the  bottom,  when  too  heavy  for  the  current 
to  lift. 

The  greater  part  of  the  debris  or  sediment  which  a  stream  car- 
ries is  furnished  to  it  by  the  destructive  activity  of  the  atmosphere  ; 
the  rains  wash  in  the  finer  materials,  while  frost  and  landslips  bring 
in  the  larger  masses  which  are  carried  down  by  mountain  torrents. 
To  this  material  the  river  adds  that  which  is  derived  from  its  own 
work  in  the  cutting  away  of  its  banks  and  bed. 

Materials  in  Solution.  —  In  addition  to  what  the  river  carries 
down  mechanically  in  suspension  or  sweeps  along  the  bottom, 
there  is  a  third  class  of  material ;  namely,  that  which  is  dissolved 
in  the  waters  of  the  stream.  Dissolved  matters  are  always  present 
in  greater  or  less  quantity,  and  are  the  same  in  kind  as  those 


IO2  RUNNING  WATER 

which  we  have  already  found  to  occur  in  spring-waters,  whence 
they  are,  for  the  most  part,  derived  by  the  rivers.  River-water 
is,  however,  usually  more  dilute  than  that  of  springs,  because  of 
the  rain  which  falls  into  it,  or  pours  in  from  the  banks.  In  very 
dry  regions,  where  this  additional  rain  supply  is  at  a  minimum, 
and  where  the  streams  are  concentrated  by  continual  evapora- 
tion, they  are  frequently  undrinkable,  on  account  of  the  quantity 
of  matters  in  solution  which  they  contain.  Examples  of  this 
are  the  salt  and  so-called  "alkali"  (a  very  comprehensive  term) 
streams  of  the  arid  West,  which  contain  a  great  variety  of  dis- 
solved minerals. 

The  quantity  of  material  which  rivers  are  continually  sweeping 
into  the  sea,  is  enormously  great.  Every  year  the  Mississippi  car- 
ries into  the  Gulf  of  Mexico  nearly  7,500,000,000  cubic  feet  of 
solid  sediment,  either  in  suspension  or  pushed  along  the  bottom, 
an  amount  sufficient  to  cover  one  square  mile  to  a  depth  of  268 
feet.  In  addition  to  this  is  the  quantity  brought  down  in  solution, 
which  is  estimated  at  2,850,000,000  cubic  feet  annually. 

Different  rivers  vary  much  in  the  proportion  of  suspended  and 
dissolved  materials  which  they  carry  and  discharge  into  the  sea ;  a 
roughly  approximate  average  makes  the  amount  of  material  removed 
equal  to  about  11,400  cubic  feet  (600  tons)  of  annual  waste  for 
every  square  mile  of  the  land  surface  of  the  globe ;  that  is,  under 
existing  conditions  of  slope,  temperature,  rainfall,  etc.  How  great 
a  difference  in  the  result  a  change  in  these  factors  may  produce, 
will  be  seen  from  a  comparison  of  the  Mississippi  and  the  Ganges. 
The  amount  of  suspended  matter  discharged  by  the  former  repre- 
sents a  lowering  of  the  surface  of  the  entire  drainage  area  at  the 
rate  of  one  foot  in  4920  years,  while  in  the  case  of  the  Ganges  it 
is  one  foot  in  1880  years,  or  more  than  twice  as  fast.  The  amount 
of  material  carried  by  the  Amazon  has  not  been  determined,  but 
there  can  be  little  doubt  that  it  is  far  greater  than  that  discharged 
by  the  Mississippi.  The  area  drained  by  the  Amazon  is  less  than 
twice  as  large  as  the  drainage  basin  of  the  Mississippi,  and  yet  it 
brings  to  the  sea  five  times  as  much  water  as  does  the  great  river 
of  North  America. 


AMOUNT  OF  LAND   WASTE  103 

The  total  amount  of  material  which  has  been  removed  from  the 
land  surfaces  by  the  atmospheric  agencies  and  carried  to  the  sea 
by  rivers  is  incalculably  great.  The  Appalachian  mountain  system 
has  thus  lost  thicknesses  of  rock  which  vary  in  different  regions 
from  8000  to  20,000  feet,  and  it  is  altogether  probable  that  the 
average  waste  of  all  the  continents  amounts  to  several  thousands 
of  feet.  The  figures  given  for  the  basins  of  the  Mississippi  and 
the  Ganges  show  that  such  wastes  imply  enormously  long  periods 
of  time. 


CHAPTER   VI 

DESTRUCTIVE   PROCESSES  — ICE,    THE   SEA,   LAKES 

Glaciers  are  much  the  most  important  form  of  ice  as  a  geological 
agent.  A  glacier  is  a  stream  of  ice  which  flows  as  if  it  were  a 
very  tough  and  viscous  fluid,  and  does  not  merely  glide  down 
a  slope,  as  snow  slides  from  the  roof  of  a  house.  Glaciers  play 
a  very  important  part  in  keeping  up  the  circulation  of  the  atmos- 
pheric waters,  and  produce  geological  results  of  an  extremely  char- 
acteristic kind.  Their  contribution  to  the  sum  total  of  rock 
destruction  and  reconstruction  is,  it  is  true,  relatively  small,  but  it 
often  becomes  important  to  trace  the  former  extension  of  glaciers, 
which,  in  its  turn,  has  a  wide  bearing  upon  some  of  the  most  far- 
reaching  of  cosmical  problems. 

As  we  ascend  into  the  atmosphere  from  any  point  on  the  earth's 
surface,  we  find  that  it  becomes  continually  colder  with  increasing 
height.  In  this  ascent  a  level  is  eventually  reached,  where  the 
temperature  of  the  air  never  rises  for  any  length  of  time  above 
the  freezing-point,  and  above  this  level  no  rain,  but  only  snow 
falls.  This  level  is  called  the  limit  of  perpetual  snow,  or  simply 
the  snow-line.  While  the  height  of  the  snow-line  above  the  sea- 
level  is,  like  climate  in  general,  much  affected  by  local  factors, 
yet,  speaking  broadly,  its  elevation  is  determined  by  latitude.  In 
the  tropics  the  snow-line  is  15,000  or  16,000  feet  above  the  sea, 
descending  more  and  more,  as  we  go  toward  the  poles,  and  coming 
down  to  sea-level  within  the  polar  circles. 

Were  there  no  means  of  bringing  the  snow  which  accumulates 
above  the  snow-line  to  some  place  where  it  may  melt,  it  would 
evidently  gather  indefinitely,  and  at  last  nearly  all  the  moisture  of 
the  earth  would  be  thus  locked  up.  As  a  matter  of  fact,  there  is 
no  such  indefinite  accumulation.  In  very  dry  regions  the  excess  of 

104 


GLACIERS  105 

snow  is  disposed  of  by  direct  evaporation,  and  on  high  mountains 
avalanches  carry  the  snow  down  to  lower  levels,  where  it  melts. 
Where  the  snow-line  is  at  sea-level,  avalanches  are  obviously  of 
no  avail.  In  places  where  the  excess  of  snow  cannot  be  disposed 
of  in  either  of  these  ways,  glaciers  are  formed  and  thus  keep  up 
the  circulation  of  the  waters,  by  carrying  the  surplus  snow  down 
to  lower  levels  at  which  it  can  melt,  or  by  entering  the  sea  and  in 
the  shape  of  icebergs  (which  are  fragments  of  glaciers)  being 
floated  to  warmer  latitudes. 


FIG.  33.  —  The  Dalton  Glacier,  Alaska.     (U.  S.  G.  S.) 

Though  even  at  the  present  time  there  are  in  various  parts  of 
the  world  great  tracts  of  glacier-ice,  they  cannot  be  called  com- 
mon and  are  found  only  where  certain  conditions  concur.  The 
nature  of  these  conditions  will  be  best  understood  by  examining 
the  process  of  glacier  formation. 

Snow  is  made  up  of  minute,  hexagonal  crystals  of  ice,  which 
are  intimately  mixed  with  air  and  thus  separated  from  one  another. 
Though  the  individual  crystals  are  transparent,  snow  is  white  and 
opaque,  as  always  results  when  a  transparent  body  is  intimately 
mixed  with  a  gas,  as  in  the  foam  on  water,  or  in  powdered  glass. 
Ice  is  composed  of  the  same  kind  of  crystals  as  is  snow,  but  they 


106  GLACIERS 

are  in  contact  with  one  another,  not  separated  by  air.  To  con- 
vert snow  into  ice,  therefore,  it  is  only  necessary  to  expel  the  air 
and  bring  the  crystals  into  contact,  for  which  pressure  alone  is 
not  ordinarily  sufficient. 

The  first  step  in  the  transformation  is  the  partial  melting  of  the 
upper  layers  of  snow,  for  which  a  change  of  temperature  is  neces- 
sary, though  the  change  need  not  warm  the  air,  but  may  be  due 
to  the  direct  rays  of  the  sun.  Glaciers  are  rare  in  the  tropics 
because  of  the  constancy  of  the  temperature,  and  the  small  area 
which  extends  above  the  snow-line,  which  seldom  permits  the  for- 
mation of  extensive  snow-fields.  Sometimes,  however,  the  condi- 
tions of  glacier  formation  are  fulfilled  even  in  the  equatorial  zone ; 
for  example,  there  is  a  glacier  on  one  of  the  peaks  of  Ecuador. 

When  the  surface  layers  of  snow  have  been  partially  melted,  the 
water  thus  formed  trickles  down  into  the  snow  beneath,  expelling 
much  of  the  air.  This  underlying  snow  has  still  a  temperature 
much  below  the  freezing-point,  and  the  percolating  water  is  soon 
refrozen  into  little  spherules  of  ice.  This  substance,  midway  be- 
tween snow  and  ice,  is  called  neve,  and  may  be  seen  every  winter 
wherever  the  snow  lies  for  any  length  of  time.  The  hardened 
"crust"  which  forms  by  the  refreezing  of  partly  melted  snow  is 
ne"ve\  The  air,  which  is  now  in  the  form  of  discrete  bubbles,  is 
largely  expelled  by  the  increasing  pressure  of  the  overlying  snow 
\  masses,  which  are  continually  added  to  by  renewed  falls,  and  the 
\  neVe"  is  thus  converted  into  ice. 

O  It  follows  from  this  that  glaciers  can  be  formed  only  where 
there  is  a  relatively  large  snow  supply,  or  at  least  where  the  snow 
accumulates  to  great  thicknesses,  and  cannot  be  disposed  of  by 
either  melting  or  evaporation.  Hence,  glaciers  are  rare  or  absent 
in  dry  regions,  where  the  snow  does  not  increase  to  great  depths, 
as  in  most  of  the  Rocky  Mountains  within  the  limits  of  the  United 
States.  It  also  follows  that  the  ground  upon  which  the  snow  lies 
must  be  so  shaped  as  to  allow  great  masses  of  it  to  gather,  with- 
out rushing  downward  in  avalanches. 

Wherever,  then,  more  snow  falls  in  winter  than  can  be  melted 
in  summer,  and  continues  to  accumulate,  glaciers  will  be  formed. 


GLACIER   MOTION 


ID/ 


A  glacier  moves  in  much  the  same  way  as  a  river,  but  at  a  very 
much  slower  rate.  The  centre  moves  faster  than  the  sides,  because 
the  latter  are  retarded  by  the  friction  of  the  banks,  and,  for  the 
same  reason,  the  top  moves  faster  than  the  bottom.  While  be- 
having like  a  plastic  substance  under  pressure,  ice  yields  readily 
to  strain,  and  even  a  slight  change  in  the  slope  of  the  bed  will 
cause  a  great  transverse  crack,  or  crevasse,  to  form,  which,  like  an 


FIG.  34. —  Crevasse  in  a  glacier,  partly  concealed  by  a  snow-bridge. 

eddy  in  a  stream,  seems  to  be  stationary,  because  always  formed 
again  at  the  same  spot.  Other  systems  of  cracks,  the  marginal 
crevasses,  are  formed  along  the  sides  of  the  glacier,  and  are  due 
to  the  more  swiftly  moving  centre  pulling  away  from  the  retarded 
sides. 

The  rate  of  glacier  movement  depends  upon  the  snow  supply, 
upon  the  slope  of  the  ground,  and  the  temperature  of  the  season. 
The  comparatively  small  glaciers  of  the  Alps  move  at  rates  varying 
from  two  to  fifty  inches  per  day  in  summer  and  at  about  half  that 


io8 


GLACIERS 


rate  in  winter,  while  the  vastly  larger  glaciers  of  the  polar  lands 
have  a  correspondingly  swifter  flow.  The  great  stream  of  ice 
which  enters  Glacier  Bay  in  Alaska  has  a  summer  velocity  of 
seventy  feet  per  day  in  the  middle. 

Southeastern  Alaska  is  a  region  where  glaciers  are  developed  on 
a  very  extensive  scale.  The  Malaspina  is  an  immense  ice-sheet, 
having  an  area  of  1500  square  miles,  which  is  formed  at  the  foot  of 

the  St.  Elias  Alps 
by  the  confluence 
of  several  great 
glaciers  from  the 
neighbouring 
mountains.  Parts 
of  this  vast  accu- 
mulation of  ice 
are  stagnant  and 
deeply  covered 
with  rock  debris, 
upon  which  there 
is  a  luxuriant 
growth  of  vegeta- 
tion, with  not  less 
than  1000  feet  of 
ice  beneath  it. 

In  Greenland 
and  the  Antarctic 
continent  the  ac- 
cumulations of  ice 
are  on  a  scale  not 
elsewhere  found,  and  these  regions  present  conditions  of  great 
geological  interest.  Greenland,  except  for  a  narrow  strip  along 
the  coasts,  is  buried  beneath  a  vast  ice-sheet,  which  can  hardly  be 
less  than  2000  or  3000  feet  thick,  and  from  which  great  glaciers 
descend  eastward  and  westward  to  the  sea.  In  the  interior  only 
a  few  isolated  mountain  peaks,  or  nunataks,  rise  through  the  ice 
mantle ;  except  for  these,  nothing  is  visible  but  illimitable  fields 


FIG.  35.  —  Vegetation  on  the  Malaspina  Glacier. 
(U.  S.  G.  S.) 


ADVANCE  AND    RETREAT  OF  GLACIERS  1 09 

of  snow.  The  snowfall  is  not  very  great ;  but  so  little  of  it  is 
disposed  of  by  evaporation  or  melting,  that  there  is  a  large  excess 
which  goes  to  the  growth  of  the  ice-sheet,  and  keeps  up  the  supply 
for  the  innumerable  glaciers  which  flow  to  the  sea. 

The  source  of  a  glacier  is  always  above  the  snow-line,  but  the 
ice-stream  itself  may  descend  far  below  that  line,  slowly  melting 
and  diminishing  in  thickness  as  it  flows.  The  lower  end  is  at  the 
point  where  the  rate  of  melting  and  the  rate  of  flow  balance,  so 


FIG.  36. —  Nunatak  rising  through   the  ice-cap,   Greenland.       (Photograph    by 

Libbey.) 

that  changes  in  the  temperature  of  the  seasons  or  in  the  amount 
of  the  snow  supply  will  cause  the  glacier  to  advance  or  retreat,  as 
one  or  other  of  these  factors  prevails.  Thus  the  Alaskan  glaciers 
have  retreated  notably  within  the  last  century,  while  some  of  the 
Norwegian  ones  are  advancing.  From  the  lower  end  of  a  glacier 
there  always  issues  a  stream  of  water,  which  flows  under  the  ice, 
often  in  great  volume,  and  even  in  winter,  for  the  thick  ice  is  a 
non-conductor  and  protects  the  stream  from  the  intense  cold 
of  the  air. 


no 


GLACIERS 


The  glaciers  mentioned  are  examples  of  the  various  forms  of 
moving  bodies  of  land  ice.  We  have  (i)  Alpine  glaciers,  of  which 
those  in  the  Alps  are  types,  and  are  relatively  small  streams  occu- 
pying narrow  mountain  valleys.  (2)  Piedmont  glaciers,  like  the 
Malaspina  of  Alaska.  These  are  great  accumulations  or  lakes  of 
ice  which  form  at  the  foot  of  mountains,  by  the  coalescence  of 


FIG.  37.  —  Edge  of  the  Greenland   ice-sheet,  with  a  glacier  descending  from  it. 
The  dark  line  is  a  medial  moraine.     (Photograph  by  Libbey.) 

numerous  glaciers  of  the  Alpine,  or  valley  type.  (3)  Continental 
glaciers  are  those  which  cover  enormous  areas  of  land,  such  as  the 
ice-sheet  under  which  nearly  all  of  Greenland  is  buried  and  that 
which  covers  the  Antarctic  land.  This  is  a  type  of  especial  inter- 
est and  significance  to  the  geologist,  because  of  the  light  which  it 
throws  upon  the  often  mysterious  operations  of  the  ice-sheets 
which  once  covered  large  portions  of  North  America  and  Europe. 
Glacier  Erosion  is  highly  characteristic,  and  enables  us  to  detect 
the  former  extension  of  ice  streams  which  have  greatly  shrunken 


GLACIER   EROSION 


I II 


and  the  former  presence  of  them  in  regions  whence  they  have 
long  vanished.  The  erosive  capabilities  of  moving  ice  have  been 
and  still  are  the  subject  of  dispute,  but  much  remains  that  can- 


FlG.  38.  —  Rock  polished  by  glacial  ice,  near  Englewood,  NJ.      (Photograph  by 

Salisbury.) 

not  be  questioned.  Thick  glaciers,  moving  with  comparative 
rapidity  down  the  steeper  slopes,  will  sweep  away  the  soil  and 
other  loose  materials  which  cover  the  ground.  The  surface  is 


FlG.  39.  —  Scored  and  smoothed  limestone  from  Montreal,  Canada. 


112 


GLACIERS 


thus,  in  the  first  instance,  rendered  more  irregular  than  before, 
because  the  hollows  formed  by  the  varying  depths  to  which 
atmospheric  disintegration  descends  (see  p.  78)  are  cleared  out 

and  the  rocks  laid  bare. 

Bare  rocks,  when  exposed  tc 
the  action  of  the  moving  ice, 
are  ground  down,  scored,  pol- 
ished, and  rounded  in  a  way 
that  can  be  accomplished  by 
no  other  agent,  and  which  is 
the  unmistakable  autograph  of 
the  glacier.  Stones  of  all  sizes, 
pebbles,  sand,  and  dust,  make 
their  way  from  the  surface  to  the 
bottom  of  the  glacier  through 
the  crevasses,  while  others  are 
picked  up  from  the  bed.  These 
hard  pieces  are  firmly  held  by 
the  great  weight  of  the  ice, 
and  are  slowly  pushed  along 
over  the  rocky  bed  with  irre- 
sistible power,  cutting  grooves 
of  a  size  corresponding  to  the 
fragments  which  do  the  work. 
Fine  particles  make  hair-like 
scratches,  large  boulders  cut 
deep  troughs,  such  as  the  re- 
markable ones  found  on  Kelly's 
Island  in  Lake  Erie ;  but  all. 
whether  coarse  or  fine,  are  in 
the  direction  of  the  glacial  flow  and  keep  parallel,  often  for 
considerable  distances. 

The  smaller  particles,  earth  and  sand,  and  those  which  are  made 
by  the  abrasion  of  the  bed,  act  as  a  polishing  powder,  and  if  the 
rocks  of  the  bed  are  sufficiently  hard,  they  will  be  finely  polished. 
Hummocks  of  rock,  over  which  the  ice  passes,  are  worn  down 


GLACIER  TRANSPORTATION  113 

and  rounded  into  the  form  called  "  rdches  moutonnees"  with  the 
side  upon  which  the  ice  impinged  gently  sloping  and  polished,  but 
with  the  down-stream  side  abrupt  and  often  not  ice  worn. 

On  a  large  scale,  glacial  erosion  produces  rounded  and  flowing 
outlines  of  hill  and  valley,  cutting  hard  and  soft  rocks  alike 
(instead  of  leaving  the  harder  standing  in  relief),  and  producing 
forms  which  are  in  marked  contrast  to  the  craggy  and  rough 
topography  of  unglaciated  regions. 

River  action  may  polish  hard  rocks  by  scouring  them  with  sand, 
but  the  glacial  furrows  and  parallel  striae  cannot  be  imitated  by 
other  agents.  To  find  these  characteristic  marks  of  ice,  it  is  not 
necessary  to  visit  actual  glaciers  ;  the  northeastern  quarter  of  the 
United  States,  from  the  Mississippi  to  the  Atlantic,  displays  them 
in  abundance,  where  harder  rocks  are  exposed  on  the  surface. 
The  rounded  forms,  the  parallel  striae,  the  polished  surfaces,  are 
common  where  the  rocks  are  hard  enough  to  retain  the  markings. 

Glacier  Transportation.  —  The  transporting  power  of  a  glacier 
is  not  determined  by  its  velocity,  at  least  so  far  as  the  material 
carried  on  its  surface  is  concerned.  This  is  because  the  rocks 
may  be  regarded  as  floating  bodies  with  reference  to  the  ice,  and 
thus  a  rock  weighing  many  tons  is  carried  with  as  much  ease  as  a 
grain  of  sand.  The  masses  of  material  transported  by  a  glacier 
are  known  as  moraines.  The  moraines  which  are  carried  on  the 
top  of  the  glacier  are  derived  from  the  cliffs  and  peaks  which 
overhang  the  ice,  and  the  action  of  frost  and  landslips  is  con- 
tinually showering  down  earth,  sand,  and  rocks  of  all  sizes,  from 
small  blocks  up  to  masses  the  size  of  houses.  This  material  is 
heaped  up  along  the  sides  of  the  glacier  in  disorderly  array,  and 
here  forms  the  lateral  moraines.  When  a  glacier  is  composed  of 
branch  streams,  it  will  have  a  corresponding  number  of  medial 
moraines  (see  Fig.  37),  in  the  middle  of  the  glacier.  When  two 
branches  unite,  their  coalesced  lateral  moraines  form  a  single 
medial  moraine. 

The  quantity  of  material  thus  carried  on  the  top  of  the  glacier 
depends  upon  the  amount  of  rock  surface  which  extends  above 
the  level  of  the  ice  and  is  subject  to  the  action  of  the  ice  and 
i 


GLACIERS 


the  atmosphere.  In  the  Alps,  where  the  glaciers  flow  in  deep 
ravines,  the  moraines  are  large,  and  some  of  the  great  Alaskan 
glaciers  have  their  lower  reaches  so  covered  with  rubbish,  that 
the  ice  is  visible  only  in  the  crevasses.  In  Greenland,  on  the 
contrary,  the  inland  ice-cap  has  very  little  material  on  its  surface, 
because  only  scattered  nunataks  rise  above  it. 


FIG.  41. —  Front  of  Bowdoin  Glacier,  Greenland.    The  dark  bands  are  made  by 
englacial  drift. 

The  substances  frozen  into  the  bottom  of  the  glacier  and  pushed 
along  over  its  bed  form  the  ground  moraine,  and  at  the  end  of  the 
glacier  is  the  terminal  moraine  (see  Fig.  57),  where  all  the  mate- 
rials carried  are  dumped  in  a  promiscuous  heap,  except  so  much 
as  is  swept  away  by  the  stream  of  water.  Besides  the  moraines 
proper,  there  is  a  certain  amount  of  englacial  drift,  carried  in  the 
body  of  the  ice.  This  is  derived  from  debris  that  comes  from  the 
surface,  but  does  not  work  its  way  entirely  to  the  bottom,  as  well 


COAST  ICE  115 

as  from  that  which  gathers  upon  the  surface  of  the  snow  or  ne"v£ 
and  is  covered  up  by  subsequent  snowfalls.  The  materials  carried 
by  a  glacier  are  as  characteristic  as  the  marks  left  upon  the  rocks 
over  which  the  ice  has  flowed.  Aside  from  the  substances  swept 
along  by  the  sub-glacial  stream,  the  various  fragments  are  not 
rounded  and  water-worn,  as  is  the  sediment  of  rivers.  The 
moraines  on  the  top  of  the  ice  (lateral  and  medial)  are  little  or 
not  at  all  abraded,  but  are  deposited  as  angular  blocks  and  frag- 
ments. The  ground  moraine,  on  the  other  hand,  is  abraded  in 
a  peculiar  way ;  the  larger  fragments  retain  their  angular  shape 
more  or  less  distinctly,  though  the  angles  and  edges  are  rounded 
off,  and  the  side  of  the  pebble  or  boulder  which  was  in  contact 
with  the  rocky  bed  is  itself  scored  and  polished.  The  finer  frag- 
ments produced  by  the  grinding  and  crushing  of  the  rocks  against 
one  another  are  angular.  In  all  this  work  of  glacial  denudation 
the  process  is  entirely  mechanical,  —  chemical  disintegration  plays 
no  part  in  it. 

Certain  other  forms  of  transportation  by  ice  may  be  conven- 
iently mentioned  here. 

Ground  Ice  forms  in  rivers  and  ponds  on  the  bottom,  freezing 
around  stones  and  boulders,  and  when  broken  up  by  thaws,  this 
ice  may  float  for  long  distances,  carrying  with  it  burdens  far 
greater  than  the  stream  which  transports  the  ice  could  carry 
unassisted.  The  shores  of  the  St.  Lawrence  River  are  fringed 
with  lines  of  large  boulders  which  have  thus  been  brought  down. 

Coast  Ice.  —  In  Arctic  regions  the  shallow  water  along  the  coast 
is  frozen  in  winter  into  a  broad  shelf  of  ice  called  the  ice-foot.  In 
the  spring  landslips  cover  the  ice  with  debris,  while  the  bottom  is 
studded  with  stones  and  pebbles.  When  the  ice-foot  is  broken  up 
in  summer,  part  of  it  is  drifted  away  and  transports  its  load  of  rock 
for  long  distances.  Other  parts  are  worked  backward  and  forward 
by  the  waves  and  tides,  scoring  the  rocks  of  the  coast  and  grind- 
ing and  polishing  the  fragments  of  rock  frozen  in  the  ice,  in  much 
the  same  fashion  as  glacial  pebbles  are  scored  and  ground.  Over 
comparatively  limited  areas  the  marks  of  coast-ice  often  have  a 
deceptive  resemblance  to  those  left  by  glaciers. 


Il6  THE   SEA 

Icebergs.  —  When  a  glacier  enters  the  sea,  it  ploughs  along  the 
bottom  until  the  buoyant  power  of  the  water  breaks  off  great  frag- 
ments of  it,  which  float  away  as  icebergs.  These  are  often  of 
gigantic  size,  veritable  islands  of  ice,  and  huge  as  they  appear, 
only  about  one-ninth  of  their  bulk  is  above  water.  As  icebergs 
are  derived  from  glaciers,  they  carry  away  whatever  debris  the 
parent  glacier  had  upon  or  within  it. 

2.   THE  SEA 

The  destructive  work  of  the  sea  is  accomplished  mainly  by 
means  of  the  waves  which  the  wind  raises  upon  its  surface. 
Ocean  currents  are,  as  a  rule,  so  far  from  shore,  and  flow  in  such 
deep  water,  that  their  erosive  power  is  comparatively  small.  The 
Gulf  Stream  is  said  to  scour  the  bottom  in  the  Florida  Straits  and 
off  the  Carolina  coast,  but  this  is  exceptional. 

Waves  act  continually  upon  all  coasts,  but  with  very  different 
force  at  different  times  and  places.  According  to  observations 
made  for  the  Scotch  Lighthouse  Board,  the  average  wave  press- 
ure on  the  coast  of  Scotland  is  for  the  five  summer  months 
6n  pounds  per  square  foot,  and  for  six  winter  months  2086 
pounds.  These  are  average  figures  and  are  greatly  exceeded  in 
storms,  when  the  force  of  the  breakers  often  rises  to  many  tons 
per  square  foot. 

The  effect  produced  by  this  great  force  depends  upon  the  char- 
acter of  the  rocks  of  the  coast,  its  height,  and  the  angle  at  which 
it  rises  out  of  the  water.  When  the  coast  is  high,  steep,  and 
rocky,  the  waves  continually  wear  away  its  base,  partly  by  dis- 
lodging the  blocks  into  which  all  consolidated  rocks  are  divided, 
and  partly  by  using  as  projectiles  the  blocks  which  it  has  dis- 
lodged, or  which  have  been  loosened  by  the  frost.  In  heavy  gales 
great  masses,  weighing  tons,  it  may  be,  are  hurled  with  tremendous 
violence  against  the  base  of  the  cliffs,  cutting  them  into  caverns, 
which  are  further  excavated  by  the  ordinary  surf.  Eventually, 
the  cliff  is  undermined,  and  the  unsupported  masses  above  fall 
in  ruins. 


EROSION  OF   ROCKY  COASTS 


117 


Waves  are  not  entirely  dependent,  as  rivers  are,  upon  the  hard 
materials  which  they  dash  upon  the  coast  for  their  efficiency  as 
destructive  agents.  The  force  of  the  mere  blow  given  by  a  storm 
breaker  is  very  great,  and  the  hydrostatic  pressure  which  first 
forces  the  water  into  every  fine  crevice  of  the  rock,  and  then 
withdraws  it,  together  with  the  sudden  compression  and  reexpan- 


FlG.  42.  —  A  rocky  shore,  coast  of  Maine.     (Photograph  by  McAllister.) 

sion  of  the  air  contained  in  these  fissures,  assists  materially  in  the 
loosening  of  the  blocks. 

Along  coasts  which  are  composed  of  hard  rocks  the  work  of 
cutting  back  the  land  by  the  sea  is  comparatively  slow,  but  when 
the  rocks  are  soft  and  yielding,  and  yet  rise  abruptly  from  the 
ocean,  the  waste  is  so  rapid  as  to  attract  every  one's  attention. 
The  coast  of  Yorkshire  in  England  is  washed  away  at  an  average 
rate  of  nearly  seven  feet  per  annum.  The  island  of  Heligoland 
near  the  German  coast,  which  has  now  a  circumference  of  less  than 
three  miles,  in  1300  A.D.  measured  forty-five  miles  around.  At 


u8 


THE   SEA 


Long  Branch,  New  Jersey,  the  sandy  bluffs  must  be  artificially  pro- 
tected against  the  attacks  of  the  sea,  yet  in  spite  of  such  protec- 
tion, almost  every  severe  gale  does  considerable  damage. 

Sandy  coasts  which  are  low-lying  and  flat  often  suffer  less  from 
the  inroads  of  the  sea  than  rocky  and  precipitous  ones,  especially 
as  they  are  apt  to  be  lines  along  which  material  is  accumulating. 
Even  such  coasts  may,  however,  be  rapidly  cut  back,  as  is  shown 


FIG.  43. 


ffects  of  marine  erosion. 


in  the  familiar  example  of  Coney  Island,  where  great  damage  has 
been  done  of  late  years.  When  the  sea  is  eating  away  a  sandy 
shore,  the  homogeneous  material  prevents  the  occurrence  of  such 
irregularities  of  the  coast-line  as  occur  in  rocky  districts.  Beside 
cutting  back  its  shores,  the  sea  continually  grinds  up  the  material 
which  is  brought  into  it  by  the  rivers,  and  that  which  it  obtains  by 
its  own  wear  of  the  coast.  The  great  blocks  on  the  shore  are 
rolled  about  in  storms,  and  worn  into  rounded  boulders,  which  are 
gradually  reduced  to  smaller  and  smaller  size.  All  the  minerals 
softer  than  quartz  are  rapidly  ground  into  fine  particles  and  swept 


CHEMICAL  ACTION  1 19 

away  by  the  undertow  into  deeper  and  quieter  waters,  leaving 
the  larger  quartz  fragments  to  form  the  pebbles  and  sand  of 
the  beach. 

The  action  of  the  waves  is  limited  vertically,  ceasing  to  be  effec- 
tive in  quite  shallow  water,  not  far  below  the  low-tide  mark.  In 
violent  storms  the  waves  often  accomplish  much  destruction  far 
above  high  tide,  but  the  principal  work  of  the  waves  is  confined  to 
a  belt  extending  from  a  little  above  high  tide  to  a  little  below  low 
tide.  Below  the  latter,  the  wave  work  is  often  efficiently  supple- 
mented by  tidal  currents,  which  under  favourable  circumstances 
acquire  great  velocity  and  depth,  scouring  away  loose  materials 
and  perhaps  even  cutting  into  solid  rock.  When  an  island  of 
considerable  extent  is  exposed  to  the  incoming  tide,  the  latter 
travels  around  the  island  in  both  directions,  and  if  the  shape  of  the 
mainland  is  favourable,  one  of  these  currents  will  be  much  higher 
than  the  other,  which  will  produce  a  "  race  "  between  the  island 
and  the  mainland.  Hell  Gate,  New  York,  is  an  example  of  this; 
the  tide  advances  through  New  York  Bay  and  Long  Island  Sound, 
being  higher  at  flood,  lower  at  ebb  in  the  sound  than  in  the  bay. 
The  consequence  is  a  swift  current  into  the  bay  at  flood  tide  and 
into  the  sound  at  ebb.  By  such  means  as  this,  the  sea  cuts  away 
the  land  to  depths  much  greater  than  unassisted  waves  can  effec- 
tively reach. 

The  chemical  disintegration  due  to  the  sea  is  not  well  marked  in 
shallow  waters,  where  the  mechanical  work  is  so  much  more  effec- 
tive and  striking.  In  the  proiound  depths  of  the  oceanic  basins, 
where  the  water  is  never  disturbed  and  where  its  motion  is 
extremely  slow,  chemical  activity  becomes  relatively  very  impor- 
tant. Calcareous  shells  are  completely  dissolved,  and  the  volcanic 
debris  which  covers  the  sea-bottom  over  vast  areas,  is  disinte- 
grated into  a  characteristic  red  clay. 

Lakes. — In  comparison  with  the  long  life  of  the  earth,  lakes 
must  be  regarded  as  merely  temporary  bodies  of  water,  which  will 
sooner  or  later  disappear,  either  by  being  drained  of  their  waters 
or  by  being  filled  up  with  the  sediments  which  are  washed  into 
them.  The  general  term  lake  is  employed  for  any  inland  body  of 


120 


LAKES 


water,  which  does  not  form  part  of  the  sea,  but  lakes  are  formed 
in  very  different  ways  and  have  correspondingly  different  histories. 
Most  lakes  occupy  depressions  below  the  general  drainage  level  of 
the  country,  whether  these  depressions  be  due  to  movements  of  the 
earth's  crust,  to  glacial  excavations,  to  unequal  decomposition  by 
the  atmospheric  agencies,  or  to  some  other  factor.  Others,  again, 
are  held  back  by  clams,  such  as  lava  streams,  glacial  moraines,  or 


FIG.  44.  —  Old  lake  terraces,  western  New  York.     (U.  S.  G.  S.) 

the  glaciers  themselves,  by  the  debris  of  landslips,  or  by  the  deltas 
of  tributary  streams  which  bring  in  more  material  than  the  main 
river  can  dispose  of.  Others  still  are  enlarged  basins  cut  out  by 
rivers.  Great  lakes  that  persist  for  long  periods  of  time  are  con- 
tained in  basins,  often  of  great  depth,  which  were  formed  by 
movements  of  the  earth's  crust ;  the  other  kinds  are  more  evanes- 
cent and  usually  of  rather  small  size. 

Small  lakes  accomplish  very  little  in  the  way  of  rock  destruction, 
but  are  rather  places  of  accumulation.     The  waves,  even  in  storms, 


LAKE   EROSION 


121 


are  not  heavy  enough  to  effectively  cut  back  the  shores,  while  the 
current  of  water  through  the  lake  is  too  slow  and  the  sediment 
transported  too  small  and  light  to  erode  the  bottom  as  a  river 
does.  In  great  lakes,  such  as  those  which  drain  into  the  St. 
Lawrence,  storms  develop  a  very  heavy  surf,  and  such  lakes 


FIG.  45.  —  Beach  on  Lake  Ontario.     (U.  S.  G.  S.) 

eat  into  their  shores  as  the  ocean  does,  but  the  very  small 
tide  confines  the  work  of  the  waves  within  narrower  limits,  and 
the  lighter  breakers  are  less  effective.  Lakes  are  subject  to 
various  accidents  which  cause  great  fluctuations  of  the  water- 
level.  Deserted  shore-lines  are  marked  by  beaches  and  terraces. 
The  method  of  denudation  by  lakes  is  the  same  as  that  of  the  sea, 
but  the  modes  of  accumulation  of  material  are  characteristically 
different. 


122  ORGANIC  AGENCIES 

ORGANIC  AGENCIES 

The  organic  agencies  are  animals  and  plants,  both  living  and 
after  death.  In  some  respects  these  agencies  tend  to  counteract 
the  destructiveness  of  others,  and  the  protective  effects  may  be 
taken  up  first. 

(1)  Protective  Effects.  —  The   protective    effects   of  organisms 
are  almost  entirely  those  of  plants,  since  animals,  on  land  at  least, 
are  not  sufficiently  abundant  to  be  of  any  importance  in  this  con- 
nection.    A  thick  covering  of  vegetation,  especially  the   elastic, 
matted  roots  of  grassy  turf,  protects  the  soil  against  the  mechani- 
cal  wash  of  rain.     How  complete  this   protection  often  is,  may 
be  seen  in  the  different  effects  produced  by  a  heavy  fall  of  rain 
upon  a  grass  field  and  on  the  adjoining  ploughed  lands,  or  even  on 
the  roads.     The  roads  may  be  so  washed  out  as  to  be  impassable, 
while  the  grass  fields  have  not  suffered  at  all.     In  certain  of  the 
western  bad  lands,  the  efficient  protection  given  by  grass  is  very 
well  shown ;    where  the  grass  has  established   itself  thickly,  the 
country  is  gently  rolling,  but  where  it  is  absent,  the  wild  and 
broken  bad  lands  are  developed. 

Vegetation,  especially  grass,  protects  loose,  light  soils  from  the 
wind,  and  often  this  is  the  only  means  by  which  sand  dunes  can 
be  held  in  place  and  prevented  from  overwhelming  valuable  lands. 
Even  the  banks  of  rivers  and  the  seacoast  may  be  efficiently  pro- 
tected by  plants.  Dense  masses  of  seaweed  growing  on  the  rocks 
form  an  elastic  buffer  against  the  surf,  and  along  low-lying  tropical 
coasts  the  mangrove  trees,  with  their  interlacing  aerial  roots,  so 
break  the  force  of  the  waves  that  they  cannot  wash  away  even 
fine  mud. 

The  only  protection  afforded  by  animals  that  requires  mention 
is  in  the  case  of  coral  reefs,  which,  thrown  up  along  or  parallel 
to  the  coast,  shield  it  from  the  heaviest  surf. 

(2)  The  Destructive  Effects  of  the  organic  agencies  are  decid- 
edly subordinate  to  those  of  the  other  classes  which  have  so  far 
been  considered,  but  they  are  not  without  importance.     We  have 
already  learned  how  greatly  the  chemical  activity  of  rain-water 


DESTRUCTIVE   EFFECTS  123 

is  increased  by  the  acids  of  vegetable  decomposition,  which  it 
absorbs  in  its  passage  through  the  soil.  Recent  observations 
show  that  the  decay  of  animals  in  the  deep  sea  is  an  agent  of  no 
mean  importance  in  promoting  the  chemical  changes  which  there 
take  place.  But  even  living  animals  and  plants  play  a  part  in 
the  work  of  disintegrating  rocks  that  should  not  be  overlooked. 
Seeds  germinating  in  the  crevices  of  rocks,  or  the  roots  of  trees 
which  invade  such  crevices  from  above,  wedge  the  rocks  apart 
with  the  same  irresistible  power  as  is  displayed  by  frost,  and  often 
large  areas  of  rock  are  thus  most  effectively  broken  up.  The 
roots  of  living  plants  also  secrete  an  acid,  which  dissolves  out 
some  of  the  soluble  constituents  of  rock,  thus  adding  a  chemical 
activity  to  the  wedge-like  mechanical  effects  of  growth. 

Many  marine  animals  bore  into  rocks,  even  the  hardest,  and 
cause  them  to  crumble,  and  on  the  land  great  numbers  of  animals 
continually  bore  and  tunnel  through  the  soil,  allowing  a  freer 
access  of  air  and  water.  In  the  tropics  the  soil  is  fairly  alive  with 
the  multitude  of  burrowers.  Earthworms  are  among  the  most 
important  agents  in  work  of  this  kind,  and  the  last  of  Mr.  Dar- 
win's books  was  a  most  interesting  one  upon  the  geological  work 
of  worms.  The  worms  swallow  quantities  of  earth,  for  the  sake 
of  the  organic  matter  which  it  contains,  and  grind  it  exceedingly 
fine  in  their  muscular  gizzards.  This  ground-up  soil  is  always 
deposited  on  the  surface,  in  the  form  of  the  coiled  "  worm-cast- 
ings," so  abundant  in  grassy  places.  Worms  are  thus  continually 
undermining  the  soil,  bringing  up  material  from  below  and  depos- 
iting it  on  the  surface,  while,  by  the  collapse  of  the  old  burrows, 
the  first  surface  gradually  sinks.  In  England  the  material  thus 
yearly  brought  to  the  surface  varies  from  seven  to  eighteen  tons 
per  acre,  which  means  an  average  annual  addition  of  one-tenth 
to  one-sixth  of  an  inch.  By  this  means  the  surface  of  the  ground 
is  constantly  changed,  and  substances,  spread  over  the  ground, 
in  the  course  of  years  make  their  way  down  into  it,  forming  well- 
defined  layers  beneath  the  surface. 


CHAPTER   VII 

RECONSTRUCTIVE  PROCESSES  —  LAND,   SWAMP,   AND    RIVER 

DEPOSITS 

WE  have  now  to  inquire  what  becomes  of  the  material  derived 
from  the  mechanical  and  chemical  disintegration  of  the  rocks,  for 
it  is  not  destroyed,  but  only  changed.  Most  of  it  is  eventually 
carried  to  the  sea  and  there  deposited ;  but  even  on  the  land,  and 
in  lakes  and  rivers,  a  certain  proportion  of  the  waste  finds  a  rest- 
ing place  for  a  longer  or  shorter  period  of  time.  While  the  rocks 
which  form  the  accessible  crust  of  the  earth  are,  for  the  most 
part,  of  marine  origin,  yet  those  formed  in  other  ways  have  great 
geological  significance,  because  of  the  assistance  that  they  give 
in  the  determination  of  ancient  land  surfaces,  lake  beds,  river 
channels,  ice  fields,  and  the  like.  Hence  it  becomes  necessary 
to  study  all  the  methods  by  which  rock  reconstruction  is  per- 
formed, on  however  small  a  scale. 

I.   TERRESTRIAL  DEPOSITS 

Under  this  head  are  included  all  those  accumulations  of  the 
mechanical  and  chemical  waste  of  preexisting  rocks,  which  are 
formed  on  land  surfaces  and  not  in  bodies  of  water.  Deposits 
made  by  ice,  on  land  or  under  water,  will  be  considered  in  a 
separate  section. 

Soil.  —  The  crumbling  remnant  of  disintegrated  rocks  forms 
soil,  which  under  the  influence  of  wind,  rain,  frost,  and  other  agen- 
cies, is  travelling  down  the  slopes,  accumulating,  often  to  great 
thickness,  in  depressions  and  valleys.  Very  little  soil,  as  such,  is 
permanently  built  into  the  earth,  by  far  the  greater  part  of  it 
reaching  the  sea  and  being  sorted  and  deposited  there.  Some- 

124 


BLOWN   SAND  125 

times,  however,  a  soil-covered  area  is  depressed  beneath  the  sea 
or  a  lake,  in  such  a  way  that  the  soil  is  not  washed  off,  but  new 
deposits  are  at  once  laid  down  upon  its  surface,  and  then  the  soil 
may  be  preserved  for  an  indefinite  period.  Such  an  old  soil,  or 
"  dirt  bed,"  may  be  recognized  by  its  texture  and  appearance  and 
by  the  roots,  stems,  and  leaves  of  land  plants,  with  which  it  is  apt 
to  be  filled.  Old  soils  are  also  preserved  in  certain  cases  by  lava 
flows,  which  have  been  poured  out  over  them. 

Talus  and  Breccias.  —  At  the  foot  of  cliffs  and  mountain  slopes, 
great  masses  of  talus,  or  angular  blocks  of  all  sizes,  accumulate, 
chiefly  through  the  action  of  frost.  These  masses  form  quite 
steep  slopes  and  show  an  imperfect  division  into  layers,  and  they 
are  continually,  but  for  the  most  part  slowly,  moving  downward, 
through  the  action  of  the  same  forces  that  produced  them.  By  the 
deposition  of  some  cementing  material  (usually  CaCO3)  the  angu- 
lar blocks  may  be  consolidated  into  a  solid  mass,  which  is  called 
breccia,  and  of  which  the  peculiarity  is  that  the  fragments  compos- 
ing it  are  angular,  not  rounded. 

Loess. — In  arid  regions  the  wind  often  carries  the  finer  parts 
of  the  soil  to  immense  distances  and  deposits  it  where  it  is  less  ex- 
posed to  the  wind,  and  where  there  is  vegetation  enough  to  hold 
it.  In  Central  Asia  the  sun  is  often  darkened  for  days  by  these 
dust-storms,  and  after  they  are  past,  a  fine  deposit  of  yellow  dust 
is  found  over  everything.  Loess  is  a  deposit  formed  in  this  way, 
and  it  is  found  in  many  lands.  One  of  the  largest  known  accumu- 
lations of  it  is  in  northern  China,  where  it  covers  an  immense  area, 
to  depths  of  1000  to  1500  feet.  It  is  not  stratified,  but  cleaves 
vertically,  and  thus  the  ravines  and  valleys  excavated  in  it  have 
very  abrupt  sides.  Loess  also  occurs  in  Europe,  and  the  Pampas 
of  the  Argentine  Republic  are  covered  with  a  great  thickness  of  it. 
The  loess  of  the  Mississippi  valley  is  believed  to  have  been  laid 
down  in  water,  under  somewhat  exceptional  conditions. 

Blown  Sand.  —  Wherever  a  sandy  soil  occurs  unprotected  by 
vegetation,  as  in  deserts  or  along  the  seacoast,  the  wind  drifts  the 
sand  and  piles  it  up  into  hills  or  sand  dunes.  The  dunes  are 
roughly  divided  into  layers,  the  thickness  and  inclination  of  which 


126 


TERRESTRIAL   DEPOSITS 


FIG.  46.  —  Sand  dune  on  the  coast  of  Rhode  Island.     (U.  S.  G.  S.) 


FIG.  47. — Sand  dune,  showing  wind  ripples.     (U.  S.  G..  S.) 


DEPOSITS   BY   SPRINGS  I2/ 

depend  upon  the  force  and  direction  of  the  wind,  and  often 
imitate  the  confused  arrangement  of  sands  piled  up  by  waves  and 
currents  under  water.  The  sand-grains  of  the  dunes  are,  however, 
more  rounded  by  the  abrasion  which  they  have  undergone,  and, 
especially  in  deserts,  they  are  apt  to  be  smaller.  When  the  sands 
are  mixed  with  pieces  of  shells  and  other  calcareous  material,  per- 
colating waters,  by  dissolving  and  redepositing  the  CaCO3,  may 
cement  the  sands  into  firm  rock.  This  is  the  more  conspicuous 
when  the  whole  material  is  calcareous,  as  in  the  shell  sands  of 
Bermuda.  This  substance,  ground  up  by  the  surf,  is  transported 
inland  by  the  wind  and  piled  up  into  dunes.  Rain-water  cements 
the  loose  grains  together,  and  by  the  alternate  accumulation  by 


FIG.  48. —  Ideal  section  through  Mammoth  Hot  Springs,  showing  the  water  rising 
through  limestone.     (Hayden.) 

wind  and  cementing  by  rain  is  formed  the  stratified  aeolian  or 
drift-sand  rock. 

Chemical  Deposits. — As  our  knowledge  of  microscopic  plants 
increases,  many  processes  which  were  believed  to  be  purely  chemi- 
cal, are  found  to  be  dependent  upon  the  activity  of  minute  plants. 
At  present,  it  is  not  possible  to  distinguish  accurately,  in  all  cases, 
between  the  two  kinds  of  processes. 

Chemical  deposits  on  the  land  are  made  principally  by  springs. 
Many  springs  precipitate  carbonate  of  lime,  on  coming  to  the 
surface.  The  quantity  of  CaCO3  which  a  given  volume  of  water 
will  dissolve,  depends  upon  the  amount  of  CO2  contained  in  that 
water,  and  the  quantity  of  dissolved  gas,  again,  is  determined  by 
the  pressure  to  which  it  is  subjected.  When  the  spring-waters 
reach  the  surface,  the  pressure  is  relieved,  much  of  the  CO2  im- 


128  TERRESTRIAL   DEPOSITS 

mediately  escapes,  and  more  or  less  of  the  CaCO3  is  deposited  as 
travertine  in  the  neighbourhood  of  the  spring,  often  in  masses  of 
considerable  extent  and  thickness.  The  process  is  not  always 
entirely  chemical.  The  beautiful  calcareous  terrace  formed  by 
the  Mammoth  Hot  Springs,  in  the  Yellowstone  Park,  is,  in  part 
at  least,  due  to  the  separation  of  the  lime  salt  from  the  water  by  a 
jelly-like  plant,  which  grows  in  the  hot  water  and  is  spread  in 


FlG.  49.  —  Travertine  terrace  of  the  Mammoth  Hot  Springs,  Yellowstone  Park. 

bright  coloured  layers  over  the  surface  of  the  terrace.  The  parts 
•of  the  terrace  where  deposition  is  no  longer  in  progress,  can  be  at 
once  distinguished  by  their  white  colour. 

Siliceous  deposits  are  much  less  common  than  the  calcareous, 
because  of  the  rare  conditions  under  which  silica  is  dissolved 
in  any  considerable  quantity,  hot  solutions  of  alkaline  carbon- 
ates being  necessary  for  this  purpose.  In  the  Yellowstone  Park, 
especially  on  the  Firehole  River,  are  great  terraces  and  flats  of 
hard  white  siliceous  sinter,  or  geyserite,  which  have  been  formed 
and  are  still  being  added  to  by  the  innumerable  hot  springs  and 


GEYSER   DEPOSITS 


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130  TERRESTRIAL   DEPOSITS 

geysers.  The  silica  is  deposited  partly  by  the  evaporation  of  the 
water  and  partly  by  the  action  of  Alga  (minute  plants)  which 
flourish  in  hot  water  pools.  Similar  deposits  are  found  in  the 
geyser  regions  of  Iceland  and  New  Zealand. 

Iron  deposits  are  formed  by  the  springs  known  as  chalybeate, 
which  contain  the  carbonate  of  iron  (FeCO3)  in  solution.  Con- 
tact with  the  air  speedily  converts  the  soluble  carbonate  into  the 
insoluble  F2O3,  which  forms  brown  stains  and  patches  on  the 
channels  leading  from  such  springs,  and  considerable  quantities  of 
it  collect  in  pools.  Here  again,  organic  agency  may  supplement 
the  chemical  work,  for  certain  diatoms  extract  iron  from  the  water, 
as  other  Algae  extract  lime  and  silica. 

Certain  mineral  springs  are  of  importance,  as  indicating  the 
way  in  which  mineral  veins  were  formed.  (See  p.  265.)  The 
Sulphur  Bank  Springs  in  the  Coast  Range  of  California -are  an 
especially  instructive  example  of  this  activity.  Below  the  depths 
to  which  the  atmospheric  influences  penetrate,  the  fissures  in  the 
rocks  are  filled  with  hydrated  silica,  which  is  as  soft  as  cheese  and 
contains  more  or  less  cinnabar  (sulphide  of  mercury).  In  other 
places  the  silica  is  hardened  to  chalcedony,  and  deposits  of  cin- 
nabar mixed  with  iron  pyrites  fill  up  the  crevices.  The  hot  waters 
which  build  up  these  deposits  are  alkaline,  charged  with  certain 
acids  and  alkaline  sulphides.  Near  Virginia  City,  Nevada,  hot 
alkaline  springs  rise  through  a  series  of  fissures,  in  which  they  have 
deposited  linings  of  silica,  amorphous  and  chalcedonic,  with  some 
quartz,  containing  minute  crystals  of  iron  pyrites  and  traces  of 
copper  and  gold.  On  the  surface  the  springs  have  formed  a  thick 
layer  of  geyserite. 

Phosphate  Deposits  are  the  only  strictly  terrestrial  organic  for- 
mations which  require  notice.  These  are  principally  derived  from 
guano,  which  is  the  accumulated  excrement  of  birds°(in  caves,  of 
bats),  and  contains  phosphates  in  large  quantity.  In  rainless 
regions,  such  as  the  Peruvian  coasts  and  islands,  the  guano  may 
accumulate  to  great  thickness  without  loss  of  its  soluble  matters, 
but  in  rainy  districts  these  are  largely  carried  away  by  percolating 
waters.  Should  the  underlying  rock  be  a  limestone,  it  will  be 


STALACTITES  1 3 1 

gradually  converted  from  a  carbonate  into  a  phosphate  of  lime. 
Such  is  believed  to  be  the  mode  of  origin  of  the  phosphatic  rock  of 
Florida  and  the  West  Indies.  On  the  other  hand,  the  phosphatic 
nodules  of  South  Carolina  are  regarded  as  due  to  the  action  of 
swamp  water  upon  underlying  shell  rocks,  though  the  source  of 
phosphoric  acid  is  not  well  understood. 

Cave  Deposits. — The  chemically  formed  cave  deposits  are  due 
to  the  solution  and  redeposition  of  carbonate  of  lime.  Caves  are 
very  generally  found  in  limestones,  and  the  percolating  waters 
which  make  their  way  through  the  roof  of  a  limestone  cavern 
always  have  more  or  less  CaCO3  in  solution.  A  drop  of  such 
water,  hanging  from  the  cavern  roof,  will  lose  some  of  its  CO2, 
upon  the  presence  of  which  the  solubility  of  the  CaCO;5  depends, 
and  deposit  a  little  ring  of  the  lime  salt.'  Successive  depositions 
will  lengthen  the  ring  to  a  tube,  and  then  the  tube  will  be  built  up 
by  layers  on  the  inner  side,  until  it  becomes  a  cone.  At  first,  the 
deposit  is  white,  opaque,  and  very  friable,  crumbling  at  a  touch, 
but  repeated  depositions  fill  up  the  interstices  of  the  porous  mass 
and  convert  it  into  a  hard,  translucent  stone,  which  assumes  a 
crystalline  structure  through  the  development  of  calcite  or  ara- 
gonite  crystals.  The  masses,  thus  formed,  that  depend  from  the 
roof  of  the  cavern,  are  called  stalactites.  After  hanging  for  a  time 
from  the  roof,  the  drop  of  water  falls  to  the  floor  of  the  cave,  and 
there,  in  similar  fashion,  deposits  a  little  layer  of  CaCO3,  which 
gradually  grows  upward  into  a  cone.  This  is  a  stalagmite,  and 
differs  from  the  stalactite  only  in  the  fact  that  it  grows  upward 
from  the  floor,  instead  of  downward  from  the  roof.  The  stalag- 
mite is,  of  course,  exactly  beneath  the  stalactite,  and  as  long  as 
the  water  continues  to  follow  the^  same  path,  the  two  cones  are 
steadily,  though  very  slowly,  increased  both  in  height  and  thick- 
ness, until  they  meet,  unite,  and  form  a  pillar  extending  from  floor 
to  roof  of  the  cavern. 

These  deposits  form  the  most  curious  and  beautiful  features  of 
limestone  caverns.  The  stalactites  assume  all  manner  of  shapes, 
determined  by  the  way  in  which  the  water  trickles  over  them,  and 
the  abundance*or  scantiness  of  the  water  supply.  Fantastic  and 


132  SWAMP   DEPOSITS 

beautiful  shapes  of  every  description,  fringes  of  crystal  spar,  and 
curtain-like  draperies  hang  from  the  roof  and  cover  the  walls  of 
the  chambers,  while  grotesque  shapes  rise  from  the  floor,  which  is 
itself  often  a  solid  mass  of  the  same  deposit,  and  the  pillars,  once 
formed,  are  ornamented  with  every  variety  of  fringe  and  sculpture. 
The  constancy  of  the  paths  by  which  the  water  descends  through 
the  roof  of  the  cavern,  insures  that  the  process  shall  continue 
uninterruptedly  for  very  long  periods  of  time.  The  Luray  Caverns 
of  Virginia  are  famous  for  the  bizarre  beauty  of  their  formations, 
but  limestone  caves  everywhere  have  more  or  less  of  the  same 
deposit  to  show. 

This  process  may  be  readily  observed  in  any  masonry  arch, 
through  which  rain-water  percolates,  as  a  bridge,  for  example. 
The  lime  of  the  mortar  is  converted,  in  course  of  time,  by  contact 
with  moist  air,  into  CaCO^,  and  this  again  is  partially  dissolved  by 
the  rain.  When  the  rain-water  trickles  through  the  arch,  it  leaves 
icicle-like  deposits,  or  thin  sheets  of  calcareous  matter,  fringing 
the  under  side. 

In  a  cave,  it  frequently  happens  that  angular  fragments  fall 
from  the  roof  and  are  cemented  into  a  breccia  by  deposits  of 
stalagmite.  In  caves  connected  with  the  surface  by  openings, 
sand  and  gravel,  or  fine  soil  and  loam,  are  washed  in  by  streams, 
or  by  the  rain,  and  form  the  characteristic  deposit  known  as  cave 
earth.  In  ancient  caverns,  no  longer  subject  to  this  wash,  the 
whole  deposit  of  earth  may  be  sealed  in  by  a  covering  of  stalag- 
mite. Cave  earth  has,  in  many  instances,  yielded  great  quantities 
of  bones,  which  were  washed  in  with  the  earth,  or  dragged  in  by 
the  carnivorous  animals  which  inhabited  the  cavern.  The  Port 
Kennedy  cave  in  Pennsylvania  is  almost  filled  up  by  the  bones  of 
extinct  animals  which  were  washed  into  it,  and  many  such  cases 
are  known,  especially  in  Europe. 

II.   PALUSTRINE  OR  SWAMP  DEPOSITS 

The  most  important  of  the  swamp  and  bog  deposits  are  the 
vegetable  accumulations,  for  the  preservation  of  which  a  certain 


GREAT   DISMAL   SWAMP  133 

amount  of  water  is  necessary.  The  vast  quantities  of  coal  which 
occur  in  so  many  parts  of  the  world,  testify  to  the  significance  of 
the  part  which  bog  and  swamp  accumulations  of  vegetable  matter 
have  played  in  the  earth's  history.  The  nearest  approach  to  coal 
that  we  have  in  process  of  formation  at  the  present  day,  we  find 
in  the  peat  bogs,  which  are  especially  abundant  and  extensive  in 
cool,  damp  climates,  as  in  Ireland,  Scandinavia,  and  the  northern 
parts  of  North  America.  In  northern  regions  the  peat  is  formed 
principally  by  mosses,  and  especially  by  the  bog  moss,  Sphagnum ; 
elsewhere,  as  in  the  Great  Dismal  Swamp  of  Virginia,  the  leaves 
of  trees  and  various  aquatic  plants  are  the  sources  of  supply. 

Vegetable  matter  consists  of  carbon,  hydrogen,  oxygen,  and 
nitrogen,  with  a  certain  proportion  of  mineral  matter,  or  ash. 
When  decaying  on  the  ground,  exposed  to  the  air,  the  plant  tis- 
sues are  completely  oxidized,  and  form  such  simple  and  stable 
compounds  as  CO2,  H2O,  NH3,  and  the  more  complex  humous 
acids,  and  thus  hardly  any  solid  residue  is  left.  In  forests  the 
accumulation  of  leaves  for  many  centuries  results  only  in  a  shallow 
layer  of  vegetable  mould.  Under  water,  where  the  supply  of 
oxygen  is  very  limited,  vegetable  decomposition  is  much  less  com- 
plete. Some  CO2,  H2O,  and  CH4  (marsh  gas)  are  formed,  but 
much  of  the  hydrogen  and  nearly  all  of  the  carbon  remain  ;  the 
farther  decomposition  proceeds,  the  higher  does  the  percentage 
of  carbon  rise,  and  the  darker  does  the  colour  of  the  mass  become. 
Peat  frequently  forms  in  small  lakes  and  ponds,  aquatic  plants 
growing  out  from  the  edges  and  on  the  surface,  until  they  gradu- 
ally fill  up  the  basin  and  convert  the  pond  into  a  bog. 

The  Great  Dismal  Swamp  of  Virginia  and  North  Carolina  prob- 
ably more  nearly  reproduces  than  do  most  existing  peat  bogs  the 
conditions  of  the  ancient  coal  swamps.  The  swamp,  which  meas- 
ures thirty  miles  by  ten,  is  a  dense  growth  of  vegetation  upon  a 
water-covered  soil  of  pure  peat  about  fifteen  feet  deep  and  with  no 
admixture  of  sediment.  The  swamp  cypress  grows  abundantly  in 
the  bog,  and  prevents,  by  its  dense  shade,  the  evaporation  which 
would  take  place  in  summer,  could  the  sun's  rays  penetrate  to  the 
wet  soil.  The  shallow  layer  of  water  which  covers  the  ground 


134 


SWAMP   DEPOSITS 


receives  the  fallen  leaves,  twigs,  and  branches,  and  sometimes 
even  the  trunks  of  fallen  trees,  preventing  their  complete  decom- 
position, while  the  dense  covering  of  mosses,  reeds,  and  ferns 
which  carpet  the  ground,  add  their  quota  to  the  mass  of  decaying 


FIG.  51.  — Great  Dismal  Swamp.     (U.  S.  G.  S.) 

vegetable  matter.  At  the  bottom  of  the  bog,  it  is  of  interest  to 
observe,  is  a  layer  of  fire-clay,  which,  by  its  imperviousness,  tends 
to  hold  the  water  and  prevent  its  draining  away.  Peat  swamps, 
formed  in  a  similar  manner,  also  occur  at  the  mouths  of  great 
rivers,  such  as  the  Mississippi. 


BOG   IRON-ORE  135 

The  bogs  of  northern  latitudes  are  due  principally  to  the  bog 
moss  Sphagnum,  which  forms  dense  and  tangled  masses  of  vege- 
tation, dead  and  decaying  below,  green  and  flourishing  above. 
As  these  mosses  hold  water  like  a  sponge,  they  will  develop  bogs 
in  any  shallow  depression,  or  even  on  a  flat  surface,  where  they 
may  get  a  foothold.  The  depth  of  peat  is  sometimes  as  much  as 
fifty  feet,  and  its  density  and  fineness  of  grain  increase  with  the 
depth  and  the  length  of  time  it  has  been  macerating  in  water. 

Fire-clay  is  frequently  found  at  the  bottom  of  peat  bogs,  and  is 
directly  connected  with  the  processes  of  vegetable  decomposition, 
though  not  itself  of  organic  origin.  Fire-clay  contains  a  large 
admixture  of  siliceous  sand,  but  is  free  from  lime,  magnesia,  the 
alkalies,  and  any  high  percentage  of  iron  ;  it  is  thus  a  mixture  of 
nearly  pure  clay  and  sand,  which  may  be  heated  very  highly  with- 
out melting  or  crumbling.  The  iron,  alkalies,  and  alkaline  earths 
are  gradually  leached  out  of  the  clay  by  the  action  of  the  peaty 
water,  which  is  charged  with  humous  acids,  and  thus  an  ordinary 
clay  is  converted  into  a  fire-clay.  Fire-clay  occurs  frequently 
beneath  coal  seams  ;  as  the  percentage  of  silica  becomes  very  high, 
fire-clay  passes  over  into  gannister,  which  is  largely  used  for  the 
lining  of  iron  furnaces. 

Bog  Iron-ore  is  another  substance  which  is  indirectly  due  to  the 
decay  of  plants ;  it  is  found  at  the  bottom  of  bogs,  or  lakes,  in 
deposits  which  are  sometimes  many  feet  thick.  Iron  is  a  very 
widely  disseminated  substance,  occurring  in  almost  all  rocks  and 
soils,  though  usually  in  very  small  quantities.  Immediately  at  or 
very  near  the  surface  of  the  ground  the  ordinary  iron  ingredient 
is  the  red  oxide  Fe2O3,  an  exceedingly  stable  compound.  In 
contact  with  decomposing  organic  matter,  and  protected  from 
the  air,  Fe2O3  is  deoxidized  and  converted  into  FeO  (Fe2O3—  O 
=  Fe2O2  =  FeO) ;  this  in  its  turn  takes  up  CO2,  which  is  present 
in  all  such  waters,  and  becomes  FeCO3,  which  is  soluble  in  carbo- 
nated water.  Solutions  of  FeCO3  accumulate  under  peat  bogs  and 
deposit  their  mineral  by  concentration  ;  but  when  the  iron-bearing 
waters  evaporate  in  contact  with  the  air,  the  carbonate  is  recon- 
verted into  the  red  oxide,  by  the  loss  of  CO2  and  absorption  of  Ov 


136  RIVER   DEPOSITS 

III.   FLUVIATILE  OR  RIVER  DEPOSITS 

In  a  preceding  chapter  we  learned  that  the  power  of  a  stream  of 
water  to  transport  sediment  depends  upon  its  velocity,  which,  in  its 
turn,  is  determined  by  the  slope  of  the  ground  and  the  volume  of 
water.  Further,  we  discovered  the  very  surprising  fact  that  the 
transporting  power  increases  as  the  sixth  power  of  the  velocity 
(T=  F6).  It  follows  from  this  that  a  slight  decrease  in  the  swift- 
ness of  a  stream  will  cause  it  to  throw  down  the  greater  part  of  its 
load  of  sediment,  while  a  slight  increase  will  cause  it  to  cany  off 
what  it  had  before  deposited.  Thus,  great  rivers,  like  the  Missis- 
sippi, which  flow  in  soft,  easily  moved  deposits,  are  preeminently 
whimsical  and  treacherous.  As  the  volume  and  velocity  of  the 
stream  are  much  subject  to  change,  there  will  obviously  be  corre- 
sponding changes  in  the  scour  and  deposition  at  any  given  point, 
but  there  are  certain  places  where  deposition  is  so  constant  that 
extensive  accumulations  may  be  formed  there.  As  we  trace  a  river 
downward  from  its  source  in  a  mountain  region,  we  find  that  in 
the  upper  stream,  which  is  a  torrent  in  swiftness,  only  large  stones 
remain  at  rest,  everything  else  being  swept  along.  Farther  down 
stream,  as  the  slope  of  the  bed  diminishes,  the  coarse  gravel  is 
thrown  down,  next  the  coarse  sand  is  deposited,  and  in  the  lower 
reaches  of  a  river,  which,  like  the  Mississippi,  flows  over  land  that 
has  a  very  gentle  slope,  and  is  raised  but  little  above  the  sea-level, 
only  the  finest  silt  gathers  on  the  bottom.  The  exact  limits  of  the 
different  kinds  of  deposit  will  vary  with  the  stage  of  water. 

At  points  where  the  velocity  of  the  stream  meets  a  constant 
check,  there  will  be  constant  deposition,  and  thus  bars  and  islands 
are  built  up  in  the  channel,  which  will  be  permanent  unless  some 
change  of  conditions  is  brought  about.  In  the  gravel  and  sand 
banks  the  material  is  stratified,  or  divided  more  or  less  distinctly 
into  layers  made  up  of  similar  fragments,  each  layer  representing 
uninterrupted  deposition.  A  pause  in  deposition  will  produce  a 
division  plane,  which  is,  of  course,  all  the  more  marked  if  the 
deposited  material  be  changed  after  the  pause.  In  the  sand  bars 
and  gravel  spits  the  up-stream  side  is  a  gentle  slope,  ending  abruptly 


FLOOD   PLAINS  137 

on  the  down-stream  side,  the  bar  or  spit  advancing  by  having  sand 
or  gravel  pushed  up  the  gentle  slope  by  the  current  and  dropped 
over  the  steep  face,  where  it  forms  inclined  layers.  Flattened  and 
elongated  pebbles  arrange  themselves  so  as  to  offer  the  least 
resistance  to  the  current,  in  a  slanting  position,  with  their  tops 
down  stream.  When  the  stream  is  subsiding,  the  material  tends 
to  assume  a  more  horizontal  direction,  giving  an  irregular  and 
confused  stratification  to  these  deposits. 

Alluvial  Cones  or  Fans.  — Where  a  swift  torrent,  descending  a 
steep  slope,  debouches  on  a  plain  or  wide  valley,  its  velocity  is 
greatly  diminished,  and  a  large  part  of  the  material  which  it  carries 
is  thrown  down  and  spread  in  a  fan  shape  from  the  opening  of  the 
ravine  in  which  the  torrent  flows.  The  thickness  of  the  cone  is 
greatest  at  the  mouth  of  the  ravine,  while  its  breadth  increases  out- 
ward from  that  point.  Where  several  such  torrents  open  on  the 
plain  near  together,  their  fans  may  coalesce  and  form  a  continuous 
fringe  along  the  base  of  the  mountain.  The  slope  of  the  cone's 
surface  diminishes  with  the  size  of  the  stream  ;  in  small  streams  it 
may  be  as  steep  as  10°.  These  cones  are  formed  on  much  the 
same  principle  as  deltas,  and  might,  with  propriety,  be  called  ter- 
restrial deltas.  Very  large  alluvial  cones  are  found  in  the  Rocky 
Mountain  and  Great  Basin  regions.  (See  Fig.  52.) 

Flood  Plains.  —  Rivers,  as  is  well  known,  are  subject  to  floods, 
when  the  volume  of  water  is  enormously  increased  and  can  no 
longer  be  contained  in  the  ordinary  channel,  but  spreads  out  over 
the  level  ground  on  each  side.  By  this  spreading,  which  may  be 
for  many  miles  in  both  directions,  the  velocity  of  the  water  is  much 
diminished,  and  over  the  flooded  area  (flood  plain)  large  quan- 
tities of  material  are  thrown  down,  while  the  unchecked  velocity 
in  the  channel  will  cause  a  scouring  and  deepening  there.  The 
nature  of  the  material  deposited  over  the  flood  plain  will  depend 
on  the  character  and  swiftness  of  the  flooded  stream,  and  varies 
from  the  coarsest  gravel  to  the  finest  silt.  The  latter  is  more 
usual,  for  the  flood  plain  is  widest  along  the  lower  course  of  the 
river.  A  line  of  maximum  deposition  occurs  where  the  swift  cur- 
rent of  the  channel  is  retarded  by  contact  with  the  sluggish  waters 


RIVER  DEPOSITS 


TERRACES  139 

of  the  flood  plain,  and  thus  the  surface  of  the  plain  slopes  away 
from  the  river.  Along  the  lower  Mississippi  this  slope  is  as  much 
as  seven  feet  for  the  first  mile.  The  vegetation,  grass,  bushes,  and 
trees,  which  grows  over  the  flood  plain,  acts  as  a  sieve  and  catches 
the  sediment,  so  that  little  escapes.  Lands  thus  annually  renewed 
by  a  deposit  of  river  mud  are  of  wonderful  fertility,  and,  like  Egypt, 
for  example,  remain  so  after  thousands  of  years  of  cultivation.  In 
this  manner  the  flood  plain  is  slowly  raised,  while,  at  the  same  time, 
the  channel  is  deepened,  and  by  the  river's  double  activity  of 
scour  and  deposition,  certain  very  characteristic  features  result. 

River  Terraces  and  Old  Gravels.  —  The  lower  courses  of  many 
rivers,  including  most  of  those  in  the  northern  United  States,  and 
some  in  the  southern,  are  bordered  by  a  succession  of  terraces  that 
rise  symmetrically  on  the  two  sides  of  the  stream.  Sometimes,  as 
in  many  English  rivers,  the  terraces  are  at  different  levels  on  oppo- 
site sides.  The  formation  of  these  terraces  is  due  to  the  twofold 
activity  of  the  river  already  described.  The  combined  deepening 
of  the  channel  and  building  up  of  the  flood  plain  at  length  make  the 
trough  of  the  river  so  deep  that  floods  no  longer  fill  it,  especially  if 
the  velocity  of  the  current  be  maintained  or  increased  by  an  ele- 
vation of  the  region  drained  by  the  river.  Then  the  energy  of  the 
current  is  partly  employed  in  widening  the  channel  and  forming  a 
new  flood  plain,  cutting  back  the  edges  of  the  old  flood  plain,  which 
it  can  no  longer  overflow,  thus  converting  it  into  a  terrace,  which  is 
the  remnant  of  an  old  flood  plain.  The  process  may  be  repeated 
many  times,  and  thus  successive  terraces  rise,  one  above  another, 
as  we  recede  from  the  river. 

It  necessarily  follows  from  this  account  that  the  highest  terrace 
is  the  oldest,  and  the  lowest  is  the  last  formed.  This  seems  to  be 
a  violation  of  the  rule  that,  in  any  series  of  sedimentary  deposits, 
the  oldest  must  be  at  the  bottom  and  the  newest  at  the  top ;  but 
the  violation  is  only  apparent,  not  real.  Were  the  river  to  flow  at 
a  constant  level,  no  terraces  could  be  formed,  and  the  deposits 
would  follow  the  rule,  just  as  they  do  now  in  each  successive  flood 
plain  and  terrace.  Because,  however,  the  stream  flows  at  suc- 
cessively lower  levels,  the  lower  flood  plain  is  made  up  of  the 


140  RIVER   DEPOSITS 

newer  deposits.  It  should  further  be  observed  that  the  older 
gravels  do  not  actually  overlie  the  newer  ones,  but  are  merely  at 
higher  levels. 

Unsymmetrical  terraces,  which  are  either  confined  to  one  side 
of  the  river,  or  if  present  on  both  sides,  are  on  different  levels,  are 
formed  when  a  stream  is  widening  its  valley  by  steadily  cutting 
away  the  bank  on  one  side,  shifting  the  channel  toward  that  side, 
and  at  the  same  time  deepening  it.  This  will  result  in  the  forma- 
tion of  terraces  representing  the  former  positions  of  the  stream. 
If  the  lateral  movement  be  all  in  one  direction,  the  terraces  will 
all  be  on  the  side  away  from  which  the  channel  is  shifting ;  if  it 
be  alternately  in  opposite  directions,  terraces  will  be  formed  on 
both  sides,  but  at  different  levels. 

Still  a  third  method  of  terrace  formation  should  be  mentioned. 
If  a  river  which  has  excavated  a  deep  valley,  have'  its  velocity 
checked  by  a  slow  subsidence  of  the  country,  it  will  commence  to 
fill  up  its  valley  with  gravel  or  other  sediment,  and  may  thus 
accumulate  material  of  great  thickness  and  extent.  Should  a  re- 
elevation  of  the  country  now  occur,  the  river  will  acquire  new 
destructive  power  and  cut  a  terraced  channel  down  through  its 
own  deposits.  In  such  a  case  the  material  is  a  continuous  mass, 
and  the  gravels  of  the  higher  terraces  are  newer  (not  older)  than 
those  of  the  lower.  The  rivers  Mersey  and  Irwell  in  England  are 
believed  to  be  examples  of  this  mode  of  terrace  formation. 

Deltas  are  accumulations  of  river  deposits  at  the  mouths  of 
streams,  land  areas  which  the  rivers  have  recovered  from  the  body 
of  water  into  which  they  flow.  The  factors  which  determine  the 
formation  of  a  delta  are  not  altogether  clear.  The  presence  or 
absence  of  a  strong  tide  is  evidently  one  of  these  factors,  for  in 
lakes  and  in  seas  with  little  or  no  tide,  almost  all  streams  form 
deltas,  while  those  rivers  which  empty  into  the  open  ocean  almost 
invariably  do  so  by  means  of  estuaries,  in  which  the  sea  encroaches 
on  the  land.  In  North  America  the  rivers  which  discharge  into 
the  Gulf  of  Mexico  form  deltas,  while  the  Atlantic  streams  nearly 
all  have  estuaries.  In  Europe  the  delta-forming  rivers  empty  into 
the  Mediterranean,  the  Baltic,  and  the  Black  and  Caspian  seas. 


DELTAS  141 

Nevertheless,  the  tide  is  evidently  not  the  sole  factor  in  deter- 
mining the  presence  of  a  delta ;  the  Thames  and  the  Rhine  dis- 
charge into  opposite  sides  of  the  North  Sea,  yet,  while  the  latter 
has  built  up  a  delta,  the  former  opens  into  a  wide  estuary.  If 
the  sea-bottom  is  subsiding  faster  than  the  river  deposit  is  built 
up,  no  delta  will  be  formed,  but  an  estuary.  The  Ganges  and 
Brahmapootra  have  formed  a  vast  delta  in  spite  of  the  powerful 
tide  of  the  Bay  of  Bengal. 

When  a  stream  loaded  with  sediment  flows  into  the  relatively 
stationary  waters  of  a  lake  or  sea,  its  velocity  is  checked  and  the 
greater  part  of  its  load  very  rapidly  thrown  down.  Deposition 
takes  place  much  more  rapidly  in  salt  water  than  in  fresh,  because 
the  dissolved  salts  reduce  the  cohesion  of  the  water,  and  hence 
diminish  the  friction  which  retards  the  settling  of  silt.  The  exces- 
sively fine  particles  of  clay,  which  in  fresh  water  remain  suspended 
for  weeks,  are  thrown  down  in  salt  water  in  a  few  hours ;  hence 
the  great  mass  of  the  sediment  falls  to  the  bottom  in  the  vicinity 
of  the  stream's  mouth.  Such  rapid  accumulation  obstructs  the 
flow  of  the  river  and  causes  it  to  divide  and  seek  new  channels, 
especially  in  time  of  flood,  and  form  a  network  of  sluggish 
streams  meandering  across  the  low  flats.  The  height  of  the  delta 
is  increased  by  the  spreading  waters  of  the  river,  when  in  flood,  and 
the  growth  of  vegetation  assists  in  raising  the  land.  Though  the 
Mississippi  delta  is  an  area  of  subsidence,  two-thirds  of  its  surface 
are  above  water,  when  the  river  is  in  its  ordinary  stages.  But  for 
the  levees,  however,  most  of  it  would  be  inundated  in  times  of  flood. 

The  nature  of  the  materials  of  which  deltas  consist  varies  ac- 
cording to  circumstances.  When  mountains  are  near  the  coast, 
the  streams  flowing  from  them  may  descend  into  the  sea  with 
sufficient  velocity  to  build  a  delta  of  cobblestones  and  coarse 
gravel.  Usually,  however,  deltas  formed  in  seas  are  composed  of 
very  fine  materials,  because  the  lower  course  of  most  rivers  is 
through  flat  plains,  and  the  stream  can  only  carry  very  fine  silt. 
Even  in  such  cases,  there  will  be  differences  in  the  coarseness  and 
fineness  of  the  material,  corresponding  to  the  seasons  of  high  and 
low  water  in  the  river. 


142  RIVER   DEPOSITS 

The  rate  of  delta  growth  is  dependent  upon  the  quantity  of 
sediment  which  the  river  brings  down,  the  depth  of  water  to  be 
filled,  the  force  of  tides  and  currents  which  scatter  the  materials, 
and  the  rate  of  subsidence  of  the  sea-bottom,  should  subsidence 
occur.  At  the  present  rate  the  Mississippi  is  pushing  its  delta  into 
the  Gulf  one  mile  for  every  sixteen  years.  The  delta  of  the  Rhone 
has  advanced  into  the  Mediterranean  more  than  fourteen  miles 
since  the  beginning  of  our  era,  while  that  of  the  upper  Rhone  in 
Lake  Geneva  has  been  built  out  one  and  one-half  miles  in  the 
same  time.  The  Po  delta  in  the  Adriatic  has  added  twenty  miles 
to  the  land  since  the  time  of  Augustus,  and  the  town  of  Adria, 
then  a  seaport,  is  now  that  distance  from  the  shore.  In  fact,  the 
whole  Adriatic  coast,  from  Trieste  to  Ravenna,  is  a  delta  forma- 
tion, which  has  widened  from  two  to  twenty  miles  since  Roman 
times.  The  Nile  delta,  on  the  other  hand,  has  advanced  very 
little  in  the  same  period,  for  a  strong  current  sweeps  along  its  sea 
front  and  carries  away  the  sediment. 

The  combined  delta  of  the  Ganges  and  Brahmapootra,  the 
two  largest  rivers  of  India,  is  interesting  as  an  example  of  a  delta 
built  up  against  a  strong  tide.  The  area  of  this  delta  is  given  as 
50,000  square  miles,  and  it  is  still  advancing,  the  rivers  deposit- 
ing more  sediment  in  flood  time  than  the  sea  can  remove  in  the 
dry  season.  The  enormous  quantity  of  material  carried  by  the 
Ganges  and  Brahmapootra,  which  far  exceeds  that  of  the  Missis- 
sippi, is  probably  the  most  important  factor  which  determines  the 
formation  of  the  delta,  despite  the  scouring  action  of  the  sea.  The 
great  rivers  of  China,  the  Hoang-ho  and  the  Yang-tze-kiang,  which 
probably  transport  more  solid  matters  than  any  other  stream  except 
the  Amazon,  have  formed  an  immense  delta  plain  around  the  head 
of  the  Yellow  Sea,  which  owes  its  colour  to  the  mud  of  these 
streams. 


CHAPTER   VIII 

RECONSTRUCTIVE   PROCESSES  —  LAKE    AND    ICE    DEPOSITS 
IV.   LACUSTRINE  OR  LAKE  DEPOSITS 

LAKES  are  important  places  of  sedimentary  accumulation,  for 
the  streams  that  flow  into  them  deposit  all  their  loads  of  suspended 
material,  and  however  turbid  the  inflowing  streams,  the  outlet  is 
clear  and  sparkling,  the  lake  acting  as  a  settling  basin.  A  most 
striking  instance  of  this  is  the  Rhone,  which  enters  Lake  Geneva 
a  muddy,  glacial  torrent,  but  flows  out  in  a  state  of  exquisite 
purity.  The  Niagara,  as  it  leaves  ,Lake  Erie,  is  likewise  beauti- 
fully clear.  Occasional  exceptions  to  this  rule  may  occur,  as  when 
a  shore  current  of  the  lake  washes  sediment  into  the  outlet,  but 
for  the  most  part,  the  lake  intercepts  and  holds  all  the  sediments 
carried  by  its  tributary  streams. 

i.  Fresh-water  Lakes,  a.  Mechanical  Deposits.  —  The  me- 
chanical sediment  which  accumulates  in  a  lake  basin  is  of  two 
kinds,  (i)  that  which  is  brought  in  by  tributary  streams,  and 
(2)  that  which  the  lake  itself  acquires  by  cutting  into  its  shores. 
In  most  cases  the  stream-borne  sediment  is  much  the  more  im- 
portant of  the  two  classes.  The  rivers  which  empty  into  lakes 
nearly  all  form  deltas ;.  if  a  strong  shore  current  sweeps  past 
the  mouth  of  the  stream,  it  will  distribute  part  of  the  materials 
along  the  shore,  and  the  waves  will  act  as  transporting  agents  to 
the  same  effect.  The  deltas  spread  out  in  a  fan-shape  from  the 
mouths  of  the  inflowing  streams,  and  if  they  are  numerous  enough, 
may  surround  the  entire  lake  with  delta  deposits.  In  each  delta 
the  successive  layers  of  sediment  will  have  an  inclination  dependent 
upon  the  depth  of  water  into  which  the  stream  debouches  and  the 
coarseness  of  the  debris  carried.  If  in  deep  water,  the  beds  may 


144 


LAKE  DEPOSITS 


O 


STRATIFICATION  145 

be  inclined  at  a  considerable  angle  ;  if  in  shallow  water,  they  form  a 
very  gradual  slope.  In  small  lakes  the  coalescence  of  deltas,  or  the 
advance  of  a  single  one,  will  eventually  fill  up  the  basin,  forming 
first  swamps  and  then  smooth,  grassy  meadows,  through  which 
flow  the  streams,  keeping  their  own  channels  clear.  Such  filled-up 
lakes  are  common  in  many  mountain  ranges.  In  large  lakes  the 
process  is,  of  course,  much  slower,  and  the  lake  is  apt  to  be  drained 
by  geographical  or  climatic  changes,  long  before  the  basin  is  choked 
with  sediment. 

Away  from  the  deltas  the  combined  action  of  the  waves  and 
currents  fringes  the  lake  with  coarse  deposits  of  boulders,  gravel, 
and  sand,  which  form  the  beach,  the  sand  extending  some  dis- 
tance out  into  shallow  water.  The  finer  materials  are  carried  out 
into  deeper  water  and  deposited  in  successive  layers  over  the 
whole  lake  bottom,  the  finest  materials  in  the  centre.  The  coarse 
and  fine  sediments  grade  into  each  other,  dovetail  and  overlap, 
because  in  heavy  storms  or  when  the  streams  are  in  flood,  the 
coarser  sediments  are  carried  farther  out  and  deposited  on  the 
fine,  and  these  changes  of  material  in  any  given  vertical  section, 
riot  too  far  from  shore,  may  be  often  repeated.  Special  lines  of 
accumulation  for  the  coarse  substances  also  occur  in  the  form  of 
shoals,  spits,  embankments,  and  the  like.  If  the  lake  is  subject  to 
fluctuations  of  its  level,  with  the  water  much  higher  at  one  time 
than  another,  even  more  wide-spread  changes  in  the  character  of 
the  deposits  will  occur.  The  deposits  now  forming  in  the  great 
Laurentian  lakes  are  principally  blue  muds  and  clays,  partly  made 
up  of  kaolinite  and  partly  of  the  debris  of  other  minerals,  in  an 
extremely  fine  state  of  subdivision,  but  not  decomposed  chemically. 
In  Lake  Superior  the  clay  has  generally  a  pinkish  tinge. 

Stratification.  —  It  has  been  incidentally  remarked  that  the 
sediment  accumulated  in  lakes  is  divided  into  layers,  which, 
except  locally,  are  laid  down  in  a  horizontal  position.  This  divi- 
sion into  layers  is  stratification,  and  the  sediment  is  said  to  be 
stratified.  Not  that  stratification  is  peculiar  to  lake  deposits  ;  on 
the  contrary,  it  is  characteristic  of  all  accumulations  made  under 
water.  It  is  due  to  the  sorting  power  of  water,  by  which,  so  long 


146  LAKE   DEPOSITS 

as  conditions  remain  the  same,  particles  of  similar  size  and  weight 
are  thrown  down  at  the  same  spot.  If  sand,  gravel,  mud,  and 
clay  be  shaken  in  a  jar  of  water  and  then  allowed  to  stand,  the 
various  materials  will  settle  to  the  bottom  in  the  order  of  their 
coarseness,  the  finest  coming  down  last.  Yet,  in  this  case,  the 
change  from  one  kind  of  material  to  another  will  be  so  gradual, 
that  no  well-defined  layers  will  appear,  to  produce  which  deposi- 
tion must  cease  at  intervals,  or  the  kind  of  material  be  changed. 
Each  layer  represents  a  time  of  deposition  followed  by  a  pause, 
which  allows  the  surface  particles  to  assume  a  somewhat  different 
arrangement  from  what  they  otherwise  would  do.  The  planes  of 
contact  between  the  successive  layers  are  called  the  bedding  or 
stratification  planes,  and  it  is  along  these  that  the  mass  of  sedi- 
ment most  readily  parts.  The  sorting  power  of  the  water  in 
a  lake  thus  causes  the  coarser  materials  to  be  thrown  down  near 
shore,  and  the  finer  to  be  carried  farther  out,  but  changes  in  the 
transporting  power,  caused  by  storms,  high  water,  and  the  like, 
change  the  place  of  deposition  for  particles  of  a  given  size,  and 
thus  pile  gravel  .on  sand,  and  sand  on  mud,,  or  vice  versa. 

Owing  to  the  way  in  which  the  materials  are  arranged,  lake 
deposits  betray  the  form  of  the  basin  in  which  they  were  laid 
down.  Around  the  old  shore  line  are  masses  of  coarse  materials, 
with  deltas  interspersed,  to  mark  the  mouths  of  streams,  while 
towards  the  middle  of  the  basin,  quantities  of  fine  mud  and  clay 
have  accumulated.  An  excellent  example  of  such  a  deserted  lake 
basin  is  that  known  as  Lake  Bonneville  in  Utah,  of  which  Salt 
Lake  is  the  shrunken  remnant.  The  drying  up  of  this  lake,  which 
was  once  fresh  and  had  an  outlet  northward  to  the  Snake  River, 
is  an  event  geologically  so  recent,  that  its  form  and  size,  its  shores 
and  islands,  its  high  and  low  stages,  in  short,  its  history,  can  be 
made  out  with  great  clearness,  as  has  been  admirably  done  by 
Mr.  Gilbert  of  the  United  States  Geological  Survey.  At  its  time 
of  greatest  extension,  Lake  Bonneville  had  an  area  of  19,750 
square  miles  and  a  maximum  depth  of  1050  feet,  while  Salt  Lake 
(which  is  variable)  had  in  1869  an  area  of  2170  miles  and  an 
extreme  depth  of  46  feet.  Around  the  ancient  shores  are  beauti- 


LAKE   BONNEV1LLE 


147 


FIG.  54.  —  Map  of  Lake  Bonneville.    The  pale  area  near  the  upper  end  marks  the 
present  outline  of  Great  Salt  Lake.     (Gilbert.) 


148  LAKE   DEPOSITS 

fully  preserved  the  terraces,  embankments,  and  deltas  of  the  various 
stages  of  water,  with  the  gravels  and  sands  appropriate  to  the  shal- 
low water.  The  principal  part  of  the  basin  is  a  level  plain,  filled  to 
a  great  but  unknown  depth  with  beds  of  clay  and  marl.  (Fig.  53.) 
In  still  more  ancient  lakes  the  terraces,  embankments,  and  other 
shore  features  have  been  swept  away  by  the  processes  of  denuda- 
tion, but  the  outline  of  the  lake  may  frequently  be  reconstructed 
from  the  character  of  the  deposits. 

b.  Chemical  Deposits  are  not  common,  nor  of  much  importance 
in  fresh   lakes.     In  a  few,  chemically  precipitated   carbonate  of 
lime  is  found,  and  more  abundant  is  limonite  (FeX),3,  H2O).     This 
is  carried  into  the  lake  by  streams  that  contain  dissolved  ferrous 
carbonate  (FeCO3),  which,  becoming  oxidized  and  hydrated,  is  no 
longer  soluble,  and  accumulates  on  the  bottom.     In  Sweden  ores 
of  this  kind  are  dredged  out  of  the  lakes  and  employed  as  a 
source  of  iron. 

c.  Organic  Deposits  are  seldom  important  in  large  lakes,  but 
often  decidedly  so  in  small  ones.     As  we  have  already  seen,  peat 
often  forms  to  such  an  extent  as  to  choke  up  the  lake  and  convert 
it  into  a  bog.     Siliceous  accumulations  are  made  on  an  extensive 
scale  by  the  minute  plants,  diatoms,  which  though  of  microscopic 
size,  yet  multiply  with  extraordinary  rapidity ;  their  tests  of  trans- 
parent flint  gather  on   many  lake   bottoms  in  a  fine  deposit,  as 
white  as  flour,  and  variously  called  Tripoli,  or  polishing  powder,  or 
infusorial    earth.     Calcareous    accumulations   are    formed   by  the 
shells  of  fresh-water  molluscs,  often  in  masses  of  considerable  thick- 
ness.    The  lower  layers  of  this  shell  marl  have  generally  been  so 
much  disintegrated  by  the  water  as  to  be  without  any  obvious  or- 
ganic structure.     Such  marls  are  frequently  found  under  peat  bogs 
and  indicate  that  the  latter  were  originally  lakes,  and  in  the  marl 
often  occur  the  bones  of  extinct  animals. 

2.  Salt  Lakes  are  especially  characteristic  of  arid  climates  in 
which  the  rainfall  is  light  and  evaporation  great.  They  may  be 
formed  either  through  the  separation  of  bodies  of  water  from  the 
sea,  or  by  the  long-continued  concentration  of  river  water  in 
basins  that  have  no  outlet,  where  the  influx  of  water  is  disposed 


CHEMICAL   PRECIPITATES  149 

of  by  evaporation  from  the  surface  of  the  lake.  In  either  case  an 
arid  climate  is  requisite  to  maintain  the  salinity;  in  a  moist  region 
the  large  rainfall  and  slower  evaporation  would  cause  the  lake  to 
rise  until  it  found  an  outlet,  and  then  the  water,  if  originally  salt, 
would  become  fresh.  The  history  of  Lake  Bonneville  exemplifies 
the  change  from  fresh  to  saline  conditions.  As  long  as  the  water 
level  was  maintained  above  the  outlet,  the  lake  was  fresh,  but  when 
the  advancing  aridity  of  the  climate  diminished  the  rainfall  and 
increased  the  rate  of  evaporation,  the  water  level  sank  until  it  fell 
below  the  outlet.  Then  the  lake  became  saline,  reaching  its 
maximum  salinity  in  the  intensely  bitter  waters  of  Salt  Lake,  which 
is  the  remnant  of  the  large  lake. 

All  river  water  contains  greater  or  less  quantities  of  dissolved 
matters,  and  of  these  one  of  the  commonest  is  ordinary  salt  (NaCl). 
When  such  waters  are  evaporated,  the  solids  remain  behind,  and 
thus  the  water  becomes  more  and  more  saline  till  it  reaches  satu- 
ration. Other  substances  occur  also,  as  will  be  seen  below. 

The  mechanical  deposits  formed  in  salt  lakes  do  not  differ  in 
any  very  important  manner  from  those  of  fresh  lakes.  The  finer 
clays  settle  more  rapidly  in  brine  than  in  fresh  water,  which  makes 
strongly  saline  lakes  extraordinarily  clear  and  limpid.  The  organic 
deposits  of  salt  lakes  are  practically  nothing,  for  brackish  water  is 
not  favourable  to  many  organisms  and  in  dense  brines  very  few 
animals  or  plants  can  exist,  and  those  that  can  are  not  the  kinds 
which  give  rise  to  peat,  or  to  siliceous  or  calcareous  deposits.  For 
the  same  reason,  the  deposits  of  whatever  kind,  laid  down  in  salt 
lakes,  are  almost  barren  of  fossils,  except  of  land  animals  and 
plants,  such  as  are  washed  into  the  lake  by  flooded  streams. 

The  chemical  deposits  are  much  the  most  interesting  and  char- 
acteristic of  the  accumulations  gathered  in  salt  lakes.  These 
chemical  precipitates  differ  much  in  the  various  lakes,  according 
to  the  nature  of  the  rocks  which  form  the  drainage  basins,  but 
while  some  of  the  substances  are  rare  and  restricted  in  extent, 
others  are  extremely  common  and  wide-spread.  Several  changing 
factors  combine  to  vary  the  order  of  precipitation  of  the  salts  in 
different  lakes,  but,  in  general,  it  follows  the  inverse  order  of 


150  LAKE   DEPOSITS 

solubility,  the  least  soluble  material  being  deposited  first  and  the 
most  soluble  last.  Comparatively  little  chemical  reaction  appears 
to  take  place  in  these  lakes ;  the  substances  are,  for  the  most  part, 
thrown  down  merely  by  the  evaporation  of  saturated  solutions  and 
are  the  same  as  those  carried  in  very  dilute  solutions  by  the  tribu- 
tary springs  and  streams.  If  the  precipitation  of  salts  is  slow 


FIG.  55. —  Island  of  calcareous  tufa,  Pyramid  Lake,  Nevada.     (U.  S.  G.  S.) 

and  occasional,  the  chemically  and  mechanically  formed  deposits 
are  mingled  together ;  but  if  such  precipitation  be  rapid,  then 
thick  and  nearly  pure  masses  of  the  salts  are  thrown  down 
in  their  proper  order,  as  the  concentration  by  evaporation 
proceeds. 

.  The  first  substances  to  be  deposited  from  solution  are  the  car- 
bonate of  lime  and  red  oxide  of  iron  (CaCO3  and  Fe2O3),  and  in 
moderately  saline  lakes  this  is  about  the  limit  of  precipitation. 
These  same  materials  are  thrown  down  in  fresh  lakes  also,  and 


CALCAREOUS   TUFA  '151 

their  deposition  is  principally  due  to  the  loss  of  the  solvent  CCX. 
The  ancient  Lake  Lahontan,  which  formerly  occupied  part  of 
northwestern  Nevada,  was  the  seat  of  calcareous  deposition  on 
a  magnificent  scale,  and  every  crag  and  island  which  its  waters 
touched  is  sheathed  in  thick  masses  of  calcareous  sinter.  Pyra- 
mid Lake,  a  remnant  of  Lahontan,  has  a  remarkable  island 


FIG.  56.  —  Calcareous  deposits  in  Mono  Lake,  California.     (U.  S.  G.  S.) 

of  calcareous  tufa ;  and  Mono  Lake,  California,  is  famous  for 
similar  deposits,  which  have  assumed  curious  and  whimsical 
shapes. 

As  the  concentration  of  the  lake  waters  proceeds,  the  next 
substance  to  be  precipitated  is  gypsum  (CaSO4,  2H2O),which, 
though  much  more  soluble  than  the  carbonate  of  lime,  is  yet  only 
sparingly  so.  After  all  the  gypsum  in  solution  has  been  thrown 
down,  there  follows  a  pause  in  the  deposition,  until  a  further  stage 
of  concentration  has  been  reached,  and  then  common  salt  is  pre- 


152  LAKE   DEPOSITS 

cipitated,  which  deposition  continues  steadily  as  concentration 
proceeds,  but  at  an  advanced  stage  the  salt  is  mingled  with  the 
sulphate  of  magnesia  (MgSO4),  should  that  be  present.  The 
highly  soluble  salts,  such  as  the  chlorides  of  magnesium  and  cal- 
cium (MgCl,  CaCl),  remain  in  solution  until  the  water  is  com- 
pletely evaporated  to  dryness,  hence  they  are  rarely  found  in  beds 
of  rock  salt. 

Various  circumstances  may  change  the  order  of  precipitation 
just  given.  In  seasons  of  high  water  the  flooded  rivers  dilute  the 
waters  of  the  lake,  checking  the  chemical  precipitation  and,  at  the 
same  time,  increasing  the  mechanical  deposition ;  thus  beds  of 
sand  and  mud  are  thrown  down  upon  the  beds  of  gypsum  and 
salt,  alternating  with  them,  as  the  influx  of  fresh  water  or  evapora- 
tion predominates.  Changes  of  temperature  also  have  an  effect 
upon  the  order  of  precipitation.  Thus,  in  cold  weather,  Salt  Lake 
washes  up  on  its  shores  quantities  of  sulphate  of  soda  (Na2SO4), 
which  is  formed  at  low  temperatures  by  the  double  decomposition 
of  NaCl  and  MgSO4. 

Besides  the  chemical  deposits  already  mentioned,  others  occur 
on  a  smaller  scale.  On  the  western  side  of  the  Great  Basin,  in 
Nevada,  California,  and  Oregon,  are  several  lakes  which  contain 
large  proportions  of  carbonate  of  soda,  and  in  some  of  them  the 
concentration  is  sufficiently  advanced  to  cause  precipitation. 

Much  the  most  abundant  of  the  chemical  deposits  made  in  salt 
lakes  are  gypsum  and  rock  salt,  and  the  enormous  scale  on  which 
the  latter  was  formed  in  past  ages  of  the  world's  history  is  demon- 
strated by  the  vast  bodies  of  rock  salt  which  are  found  embedded 
in  the  rocks  in  so  many  parts  of  the  world.  Near  Berlin,  at 
Sperenberg,  an  artesian  well  was  sunk  through  such  a  deposit  for 
nearly  4000  feet,  without  reaching  the  bottom.  In  various  regions 
of  the  United  States,  notably  in  New  York  and  Kansas,  large 
bodies  of  salt  are  found,  but  not  on  such  a  scale  as  in  Europe. 

It  should  be  noted  that  the  chemical  deposits  made  in  salt  lakes 
are  crystalline  and  at  the  same  time  stratified.  This  association 
is  not  the  usual  one,  as  stratified  rocks  are  ordinarily  not  crystal- 
line, and  crystalline  rocks  are  mostly  unstratified. 


GLACIAL   DEPOSITS 


153 


V.    ICE  DEPOSITS 

Deposits  made,  directly  or  indirectly,  by  the  agency  of  ice  are 
very  characteristic,  and  though  some  are  formed  on  land  and  some 
under  water,  it  is  desirable  to  consider  them  together  in  a  single 
section.  The  peculiar  features  of  ice  formations  may  be  much 
obscured  by  the  action  of  water,  either  at  the  time  of  their  deposi- 
tion or  at  some  subsequent  period.  Ice  deposits  play  but  a  very 


FlG.  57.  —  Glacier  des  Bossons,  Switzerland.    The  terminal  moraine  follows  the 
lower  end  of  the  glacier.     (Photograph  by  McAllister.) 

small  part  in  the  construction  of  the  earth's  crust,  but  the  light 
which  they  throw  upon  changes  of  climate  and  similar  questions, 
lends  them  an  unusual  degree  of  interest. 

Glacial  Deposits. — We  have  already  learned  that  glaciers  carry 
with  them  great  masses  of  debris,  either  in  the  form  of  lateral 
and  medial  moraines  upon  their  upper  surfaces,  or  frozen  in  the 
interior  of  the  ice,  or  pushed  along  beneath  it.  When  the  glacier 
arrives  at  its  lower  end,  where  the  rates  of  motion  and  melting 


154 


ICE   DEPOSITS 


balance  each  other,  all  the  burden  which  it  is  transporting  is 
deposited  in  a  great  mound  or  ridge,  the  terminal  moraine. 
Moving  ice  does  not  sort  the  material  which  it  carries,  as  flowing 
water  does,  because  in  a  glacier  there  is  no  such  definite  relation 
between  velocity  and  transporting  power.  Hence,  the  terminal 
moraine  is  unstratified  and  is  composed  of  materials  of  all  sizes, 
from  dust  and  sand  up  to  great  boulders  weighing  hundreds  of 
tons,  all  mingled  together  in  confusion.  In  the  case  of  a  glacier 


FlG.  58.  —  Perched  block  of  sandstone  resting  on  trap,  Palisade  Ridge,  NJ. 
(Photograph  by  Salisbury.) 

which  carries  the  principal  part  of  its  burden  upon  its  upper  sur- 
face, the  terminal  moraine  is  chiefly  made  up  of  angular  blocks 
that  have  undergone  little  or  no  abrasion,  together  with  earth, 
sand,  gravel,  and  whatever  kind  of  material  the  overhanging  cliffs 
may  have  delivered  to  the  moving  ice.  Mingled  with  these 
materials,  however,  will  be  found  more  or  fewer  of  the  character- 
istically worn  and  grooved  glacial  pebbles  and  boulders,  which 
have  been  dragged  along  under  the  ice,  and  scored  and  polished 
by  the  rocky  bed.  There  will  also  be  found  some,  at  least,  of 
the  sand  and  line  rock  flour  which  the  glacier's  own  movement 


ERRATIC  BOULDERS  155 

produces  and  which  have  escaped  the  washing  of  the  sub-glacial 
stream. 

When  a  glacier  is  retreating,  it  may  build  up  a  new  terminal 
moraine  at  each  point  of  arrested  withdrawal,  or  if  the  retreat  is 
gradual  and  steady,  the  ground  in  front  of  the  ice  will  be  covered 
with  moraine  material,  spread  out  in  a  sheet,  not  heaped  up  in  a 


FIG.  59. — Perched  block  near  the  Yellowstone  Canon,  National  Park. 
(U.  S.  G.  S.) 

moraine  or  mound.  The  retreat  of  the  glacier  may  leave  behind 
it  isolated  masses  of  ice  deeply  buried  in  the  debris  of  the  terminal 
moraine ;  when  such  masses  melt  they  form  depressions  in  the 
mound  and  give  rise  to  the  "kettle  moraines."  A  shrinking  glacier 
will  contract  laterally  and  in  depth,  as  well  as  longitudinally,  and 
in  this  way  the  blocks  of  the  lateral  moraine  will  be  left  stranded  at 
intervals  over  the  former  glacial  bed.  Such  blocks  and  boulders 
are  known  as  erratics,  or  perched  blocks,  and  when  their  parent 


156  ICE  DEPOSITS 

ledge  can  be  discovered,  it  is  easy  to  determine  the  distance  to 
which  they  have  been  carried.  Sometimes  a  great  boulder  is 
lowered  so  gradually  and  gently  by  the  retreating  ice,  that  it  is 
exactly  balanced,  and  may  be  moved  backward  and  forward  by  the 
hand.  This  is  a  "  rocking-stone,"  though  it  must  not  be  supposed 
that  all  rocking-stones  are  glacial.  (See  p.  86.) 

The  water  deposits  which  are  made  in  the  neighbourhood  of  and 
in  association  with  a  glacier,  are  also  characteristic  and  should  be 


FlG.  60. —  River  issuing  from  the  Malaspina  Glacier,  Alaska.     (U.  S.  G.  S.) 

noticed  in  this  connection.  Very  instructive  examples  of  this 
combined  action  may  be  observed  about  the  great  Malaspina 
glacier  in  Alaska.  This  is  an  immense  ice-sheet,  with  an  area 
of  1500  square  miles,  which  is  formed  at  the  foot  of  the  St. 
Elias  Alps  by  the  confluence  of  several  great  glaciers.  All  the 
outer  borders  of  the  glacier  are  covered  with  sheets  of  moraine 
matter,  and  upon  the  stagnant  portion  of  this  is  a  luxuriant  growth 
of  bushes,  beneath  which  is  a  thickness  of  not  less  than  1000  feet 


STRATIFIED    ICE   DEPOSITS 


157 


of  ice.  About  the  margin  of  the  ice-sheet,  small  lakes  are  formed, 
the  water  being  held  in  place  by  the  ice  barrier,  but  these  lakes 
are  subject  to  great  fluctuations,  and  often  their  waters  escape 
through  tunnels  in  the  ice.  In  some  of  these  lakes  stratified 
deposits  are  made  by  the  inflowing  streams.  Innumerable  streams, 
some  of  them  quite  large,  rise  from  under  the  glacier,  and  many 
others  flowing  from  the  north  pass  under  the  free  margin  of  the  ice 
by  means  of  long  tunnels.  All  of  these  streams  are  loaded  to  their 


FIG.  61.  —  The  Chaix  Hills,  Alaska.     Moraine  material  stratified  by  water. 
(U.  S.  G.  S.) 

utmost  capacity  with  sediment,  gravel,  and  boulders ;  by  blocking 
up  their  own  openings  from  the  ice,  they  likewise  cause  the  deposi- 
tion of  sand,  gravel,  and  boulders  within  their  tunnels,  which,  when 
the  glacier  retreats,  will  be  left  standing  as  the  gravel  ridges  called 
asars,  while  conical  mounds  are  built  up  where  the  streams  burst 
from  under  the  ice,  and  sometimes,  owing  to  the  great  pressure, 
rise  like  fountains.  This  kind  of  deposition  is  characteristic  of 


158  ICE   DEPOSITS 

retreating  ice-sheets,  such  as  the  Malaspina ;  in  advancing  glaciers 
denudation  will  prevail  over  deposition. 

Iceberg  Deposits. — When  a  glacier  flows  into  the  sea,  it  con- 
tinues to  advance  until  the  buoyant  power  of  the  water  is  suf- 
ficient to  raise  and  float  it  :  as  ice  can  endure  very  little  strain, 
great  masses  are  thus  broken  off  and  float  away  as  icebergs.  Ice- 


FlG.  62. — Deposit  partly  made  by  stranded  ice,  west  coast  of  Greenland. 
(Photograph  by  Libbey.) 

bergs  are  thus  seen  to  be,  as  indeed  they  always  are,  derived  from 
land  ice  and  not  from  the  freezing  of  sea-water.  The  iceberg  will, 
of  course,  carry  with  it  whatever  parts  of  the  glacial  debris  are 
contained  within  or  upon  that  particular  fragment  of  the  glacier, 
and  drops  this  load  over  the  sea-bottom,  as  the  berg  gradually 
melts.  As  the  Greenland  icebergs  sometimes  drift  as  far  south  as 
the  Azores,  glacial  boulders  are  scattered  all  over  the  bed  of  the 
North  Atlantic,  and  thus  we  see  how  large  blocks  may  be  em- 


COAST  ICE 


159 


bedded  in  stratified  deposits  very  far  from  the  place  where  they 
were  torn  from  their  parent  ledges. 

Coast  Ice  Deposits.  —  In  high  latitudes  with  intensely  cold  winters, 
great  fields  of  ice  (the  ice-foot)  are  formed  by  the  freezing  of  sea- 
water  along  the  shore.  The  ice-foot  becomes  loaded  with  great 
masses  of  rock,  part  of  which  is  thrown  down  from  overhanging 
cliffs  by  the  action  of  frost,  part  picked  up  from  the  shore-line  by 
the  ice  forming  around  it.  In  summer  the  coast  ice  breaks  up  and 
floats  away  with  its  load  of  blocks  and  boulders,  distributing  them 
over  the  sea-bottom  just  as  icebergs  do.  In  storms  great  masses 
of  coast  ice  are  often  driven  on  the  shore,  where  they  may  pile  up 
to  heights  of  fifty  feet  or  more,  carrying  some  of  the  boulders 
above  the  levels  at  which  they  were  picked  up.  The  coast  of 
Labrador  is  covered  for  long  distances  with  boulders  thus  trans- 
ported, as  are  many  other  Arctic  shores.  Great  masses  of  rock 
are  thus  transported  in  the  Baltic,  and  the  divers  report  that  in 
the  Copenhagen  Sound  the  sunken  wrecks  of  vessels  are  covered 
with  ice-borne  blocks. 


CHAPTER   IX 

RECONSTRUCTIVE  PROCESSES  —  MARINE   AND  ESTUARINE 
DEPOSITS 

VI.     MARINE  DEPOSITS 

THE  sea  is  the  great  theatre  of  sedimentary  accumulation,  and 
rocks  of  marine  origin  form  by  far  the  largest  part  of  the  present 
land  surfaces.  Important  as  other  classes  of  deposits  may  be,  all 
of  them  together  are  very  much  less  so  than  those  laid  down  in 
the  ocean  and  the  waters  immediately  connected  with  it.  There 
is  great  variety  in  the  sedimentary  deposits  made  in  the  sea,  which 
change  in  accordance  with  the  depth  of  water,  the  nature  of  the 
coast  rocks,  the  force  of  winds  and  tides,  and  the  nearness  or 
remoteness  of  the  mouths  of  rivers.  Large  land-locked  seas,  like 
the  Gulf  of  Mexico  and  the  Mediterranean,  again,  have  deposits 
more  or  less  different  from  those  of  the  open  ocean,  a  difference 
which  is  largely  due  to  the  absence  or  insignificance  of  the  tide, 
and  the  reduced  force  of  the  waves. 

It  is  important  to  remember  that  the  actual  line  of  meeting  of 
sea  and  land  is  not  the  structural  margin  of  the  continent,  for  the 
water  may  cover  a  broad  submerged  shelf  of  the  latter.  For  100 
miles  east  from  the  coast  of  New  Jersey  the  water  deepens  very 
gradually  to  the  100- fathom  line,  whence  it  shelves  very  steeply 
to  the  profound  oceanic  abyss.  The  loo-fathom  line,  which  may 
be  far  out,  or  close  inshore,  generally  represents  the  true  margin 
of  the  continental  platform.  ?  (See  Fig.  63.) 

Marine  deposits  may  be  classified  primarily  in  accordance  with 
the  depth  of  water  in  which  they  were  laid  down,  one  of  the  most 
valuable  guides  to  the  history  of  ancient  rocks,  and  secondarily  in 
accordance  with  the  nature  of  the  material  of  which  they  are  com- 

160 


CLASSIFICATION   OF   MARINE   DEPOSITS 


161 


posed,  and  the  processes  by  which  they  were  accumulated.  The 
classification  proposed  by  Murray  and  Renard  from  a  study  of 
the  great  collections  of  modern  marine  deposits  made  by  the 
"Challenger"  expedition  is  as  follows:  — 


Littoral  Deposits,  be- 
tween high  and  low 
water  marks. 


MARINE  DEPOSITS 


Sands,  gravels,  muds, 
etc. 


2.  Shallow-water  Deposits  1 

between      low-water  !  Sands,  gravels,  muds, 


mark   and 
oms. 


100  fath- 


3.  Deep-sea  Deposits  be- 
yond 100  fathoms. 


etc. 


r  Coral  Mud. 
Volcanic  Mud. 
Green  Mud  and  Sand. 
Blue  Mud. 
Red  Mud. 

Foraminiferal  Ooze. 
Pteropod  Ooze. 
Diatom  Ooze. 
Radiolarian  Ooze. 
Red  Clay. 


I.  Terrigenous  Deposits 
formed  in  deep 
and  shallow  water, 
close  to  land 


II.  Pelagic  Deposits, 
formed  in  deep 
water  removed 
from  land. 


The  material  brought  into  the  sea  by  rivers,  or  washed  from  the 
shore  by  waves,  is  partly  mechanically  suspended  and  partly  dis- 
solved ;  the  former  will  be  deposited  when  the  moving  water  is  no 
longer  able  to  transport  it,  while  the  latter  is,  to  a  large  extent, 
extracted  from  solution  by  the  agency  of  animals  and  plants, 
though  some  of  it  remains  permanently  dissolved.  The  sorting 
power  of  water,  which  is  as  conspicuous  in  the  sea  as  in  lakes 
or  rivers,  arranges  the  mechanically  borne  sediments  according  to 
the  coarseness  and  fineness  of  their  constituent  particles,  at  the 
same  time  effecting  a  rough  separation  of  the  materials  accord- 
ing to  their  mineralogical  composition.  Marine  deposits  are  thus 


162 


MARINE  DEPOSITS 


always  stratified,  though  in  cases  when  deposition  is  continued 
for  long  periods  without  interruption,  thick  masses,  not  obviously 
divided  into  layers,  may  be  accumulated,  but  this  is  exceptional  in 
those  parts  of  the  sea  where  deposition  is  most  rapid. 

i.  Littoral  Deposits  are  laid  down  between  high  and  low  tide 
marks  and  a  little  beyond  the  low-water  mark,  in  very  shallow 
water.  These  are  made  up  of  the  coarsest  materials,  boulders, 


FIG.  63.  —  Basin  of  the  Gulf  of  Mexico,  showing  the  submerged  margin  of  the 
continental  platform  and  the  steep  descent  of  the  bottom  at  the  loo-fathom  line. 
Vertical  scale  much  exaggerated.  (From  a  model  by  the  U.  S.  Coast  Survey.) 

coarse  gravel,  and  sand,  though  mud  may  accumulate  in  holes 
and  sheltered  situations,  even  within  this  zone.  The  principal 
grinding  action  of  the  surf  is  exerted  between  tide  marks,  and  the 
undertow  and  tidal  currents  continually  sweep  out  the  finer  parti- 
cles to  deeper  and  quieter  water,  leaving  the  co3rser  fragments 
behind.  Mineralogically,  these  coarser  fragments  may  be  of  any 
kind,  depending  upon  the  rocks  of  the  coast  and  the  material  sup- 
plied by  neighbouring  rivers,  but  most  frequently  they  are  quartzose, 
which  is  due  to  the  superior  hardness  of  that  mineral. 


LITTORAL  ACCUMULATIONS 


163 


Littoral  deposits  are  thickest  toward  the  shore,  thinning  out  to 
an  edge  seaward,  where  they  dovetail  in  with  finer  materials,  be- 
cause the  coarser  fragments  are  carried  farther  out  at  some  times 
than  at  others.  Where,  for  long  distances,  no  large  rivers  enter 
the  sea,  the  materials  are  all  derived  from  the  wear  of  the  coast, 
and  the  distribution  of  coarse  and  fine  deposits  is  more  regular  and 
uniform,  and  gravel  beds  may  extend  as  far  as  ten  miles  from  the 


FIG.  64.  —  Littoral  deposits  on  the  west  coast  of  Greenland.      The  angular  pieces 
transported  by  ice.     (Photograph  by  Libbey.) 

coast.  The  regularity  of  arrangement  is  interfered  with  by  con- 
flicting currents,  and  in  eddies  of  quieter  water  will  be  found  areas 
of  finer  deposits.  Beds  laid  down  in  very  shallow  water  are  apt 
to  be  characterized  by  irregularities  of  stratification,  known  as 
false  or  current  bedding ;  ripple  marks  and  tracks  of  animals  are 
other  indications  of  the  immediate  proximity  of  the  shore. 

It  will  at  once  be  evident  that  no  great  thickness  of  shallow- 
water  deposits  can  be  formed,  unless  the  sea-bottom  is  sinking, 
because,  otherwise,  the  water  would  soon  be  filled  up  and  deposi- 


164  MARINE  DEPOSITS 

tion  cease  at  that  point.  If  the  rate  of  subsidence  be  very  slow, 
deposition  may  shoal  the  water  and  thus  extend  the  seaward  range 
of  the  coarse  materials,  forming  broad  belts  of  them,  while,  if  the 
rates  of  deposition  and  sinking  be  nearly  equal,  the  coarser  de- 
posits will  be  restricted  to  long  and  narrow  bands,  running  paral- 
lel with  the  coasts.  If,  on  the  other  hand,  subsidence  takes  place 
at  a  rapid  rate,  the  water  will  be  deepened,  the  coast  will  retreat, 
and  where  coarse  materials  were  before  gathered,  only  the  finer 
will  now  be  accumulated.  Thus  in  the  same  vertical  line  will  be 
formed  rocks  which  indicate  very  different  depths  of  water. 

2.    Shallow-water  Deposits.  —  Beyond  the  low-tide  mark,  and 
for  a  distance  out  to  sea  which  depends  upon  the  slope  of  the 


FlG.  65.  —  Diagram  illustrating  the  change  of  materials  on  the  sea-bottom  and 
the  dovetailed  edges  of  sandy,  clayey,  and  calcareous  beds.  Owing  to  the  great  exag- 
geration of  the  vertical  scale,  the  beds  appear  unduly  irregular. 

bottom,  the  sea-bed  is  ordinarily  covered  with  sand.  If,  as  along 
the  eastern  coast  of  the  United  States,  the  water  deepens  very 
gradually  and  the  loo-fathom  line  is  far  from  the  shore,  sand  will 
be  found  100  to  150  miles  out,  growing  finer  and  finer  with  the 
increasing  depth.  Throughout  this  whole  belt  wave  action  is 
exerted  on  the  bottom,  though  to  a  very  insignificant  extent  in 
the  deeper  parts,  but  the  stratification  is  regular  and  uniform,  and 
the  materials  are  widely  and  evenly  distributed.  The  characteristic 
deposit  of  this  zone  is  siliceous  sand,  though  under  exceptional 
circumstances,  considerable  areas  of  muds  and  clays  may  be 
formed.  Thus,  south  of  Block  Island  is  a  large  triangular  patch 
of  clay  which  invades  the  sand  area,  and  several  large  mud  holes 
occur  off  the  entrance  to  New  York  Bay.  The  action  of  the  waves 
and  shore  currents  transports  the  sand  along  the  coast  for  long 


ACCUMULATION   IN   SHALLOW  WATER  165 

distances  from  its  place  of  origin.  An  instance  of  this  is  the 
Atlantic  coast  of  Florida,  which  has  a  siliceous  sand  belt  that 
cannot  have  been  derived  from  the  peninsula. 

Organic  deposits  are  much  less  common  in  shallow  water  than 
are  the  terrigenous,  and  yet  under  favourable  conditions  they  are 
developed  on  a  very  extensive  scale.  The  most  important  of  such 
conditions  are  warm  water  and  the  presence  of  ocean  currents 
which  bring  abundant  supplies  of  food.  The  sea  is  constantly 
receiving  from  the  land  materials  in  solution,  of  which  the  most 
important  are  the  carbonate  and  sulphate  of  lime.  Many  classes 
of  marine  animals  extract  the  CaCO3  from  the  sea-water  and  form 
it  into  hard  parts,  either  as  external  shells  and  tests,  or  as  internal 
skeletons.  There  is  also  good  reason  to  believe  that  some,  at 
least,  of  these  organisms  are  able  to  convert  the  sulphate  into  the 
carbonate. 

The  classes  of  marine  organisms  which  at  present  or  in  times 
past  have  played  the  most  important  part  in  the  accumulation  of 
calcareous  material  are  :  the  Foraminifera,  Corals,  Echinoderms, 
and  Molluscs  ;  but  other  groups  contribute  extensively  to  the  same 
result.  The  Foraminifera  do  not  accumulate  with  sufficient  rapid- 
ity to  add  largely  to  the  calcareous  deposits  of  shallow  water,  and 
will  therefore  be  considered  in  connection  with  the  deep-sea  for- 
mations. 

Corals. — The  animals  of  this  group  are  of  many  varieties  of 
form,  size,  and  habit,  and  by  no  means  all  of  them  are  important 
as  rock- makers.  The  solitary  corals,  for  example,  are  widely  dis- 
tributed in  the  deep  sea,  but  are  never  sufficiently  abundant  to 
form  deposits  by  themselves.  Those  corals  which  do  accumulate 
in  great  masses,  and  are  known  as  reef-builders,  form  compound 
colonies  or  stocks,  in  which  hundreds  or  thousands  of  individuals 
are  united.  The  adult  corals  are  stationary,  but  the  newly  hatched 
young  are  worm-like,  free-swimming  larvae.  When  the  young  ani- 
mal establishes  itself  in  a  suitable  place,  it  develops  into  a  polyp,  or 
fleshy  sac,  with  a  mouth  surrounded  by  rows  of  tentacles,  and  then 
by  budding  or  partial  division  (fission)  gives  rise  to  great  numbers 
of  other  polyps,  which  are  connected  together  by  a  tissue  common 


1 66  MARINE   DEPOSITS 

to  them  all.  In  this  compound  mass  is  secreted  a  skeleton  of 
carbonate  of  lime,  which  reproduces  the  form  of  the  colony  and, 
in  most  cases,  displays  cells  for  the  individual  polyps.  The  great 
variety  of  form  shown  by  these  compound  colonies  is  determined 
by  the  mode  of  budding  or  fission  and  the  relative  position  of  the 
newer  to  the  older  polyps.  Thus,  some  are  like  trees,  others  like 
bushes ;  some  form  flat,  irregular  plates,  while  others  grow  into 
great  dome-like  masses. 

The  reef  corals  have,  at  present,  a  restricted  distribution,  and 
can  flourish  only  where  several  favourable  conditions  are  found 
united.  They  are  preeminently  shallow- water  animals  and  can  live 
only  in  depths  of  less  than  twenty  fathoms.  They  also  require  a 
high  temperature,  and  they  cease  wherever  the  average  tempera- 
ture of  the  water  for  the  coldest  month  is  below  68°  F. ;  this  is  the 
minimum,  and  for  full  luxuriance  a  higher  temperature  is  neces- 
sary. Another  requisite  is  sea-water  of  full  salinity  and  uncontami- 
nated  with  mud  ;  hence,  corals  cannot  live  at  the  mouth  of  a  river, 
which,  even  if  it  brings  down  no  sediment,  freshens  the  water  and 
is  thus  fatal  to  the  polyps.  Another  condition  favourable  to  the 
growth  of  corals  is  the  presence  of  ocean  currents,  not  too  rapid, 
which  bring  abundant  supplies  of  food,  and  they  flourish  best  in 
the  broken  waters  of  heavy  surf,  which  gives  the  necessary  oxygen 
and  prevents  the  smothering  of  the  polyps  in  the  calcareous  silt 
and  debris  of  the  reef.  In  short,  the  reef  corals  are  tropical, 
marine,  shallow-water  animals,  and  their  reefs  are  widely  spread 
throughout  the  warmer  seas  of  the  globe,  but  they  do  not  always 
occur  where  we  should  naturally  expect  to  find  them. 

A  coral  reef  is  not  built,  as  many  people  imagine,  by  the  indus- 
try of  the  polyps  —  these  furnish  the  material,  by  extracting  lime 
salts  from  the  water  and  forming  solid  skeletons ;  the  actual 
construction  is  largely  the  work  of  the  waves,  for  the  corals  live 
within  the  limits  of  wave  action.  The  coral  colonies  are  scattered 
over  the  sea-bottom,  much  like  vegetation  on  the  land,  scantily  in 
some  places,  thickly  in  others,  and  in  still  others  they  are  absent. 
The  waves,  especially  in  storms,  break  up  the  masses  of  coral, 
which  are  much  weakened  by  the  borings  of  many  kinds  of  marine 


CORAL   REEFS 


I67 


animals,  and  the  surf  grinds  them  down  to  fragments  of  all  sizes, 
from  large  blocks  to  the  finest  and  most  impalpable  mud.  The 
process  is  the  same  as  with  the  ordinary  rocks  of  the  coast,  only 
the  material  differs,  and  thus  are  formed  boulders,  pebbles,  sand 
and  mud,  all  of  coral  fragments.  The  many  animals  which  feed 
upon  coral  greatly  facilitate  this  work,  partly  by  boring  into  the 


FIG.  66.  — Patch  of  corals  on  the  Great  Barrier  Reef  of  Australia.     (Savile  Kent.) 

masses,  partly  by  grinding  the  smaller  fragments  into  fine  powder. 
Considerable  masses  of  calcareous  debris  are  added  by  the  shells 
and  tests  of  the  various  animals  which  live  in  and  about  the  reef, 
and  the  coral-like  seaweeds,  called  Null'pores,  contribute  an  im- 
portant quota.  All  of  this  material  is  ceaselessly  ground  up  by 
the  waves,  distributed  by  tides  and  currents,  and  brought  to  rest  in 
quiet  waters.  A  single  deposit  of  two  or  three  inches  in  thickness 
has  been  observed  to  form  between  tides,  after  a  gale  along  the 


1 68 


MARINE   DEPOSITS 


Florida  reefs,  and  in  storms  the  water  is  often  discoloured  and 
turbid  for  miles  around  the  reef.  The  sea-water  dissolves  and 
redeposits  CaCO3,  cementing  the  fragments  into  a  firm  rock, 
which,  especially  after  exposure  to  the  air,  may  become  very  hard. 
By  these  processes  several  varieties  of  rock  are  formed,  corre- 
sponding, in  all  but  the  material,  to  the  ordinary  marine  deposits. 
In  one  form  the  standing  and  unbroken  colonies  are  filled  up  with 
calcareous  debris  and  enclosed  in  solid  masses.  Coral  conglomer- 


FlG.  67. — Corals  on  the  Great  Barrier  Reef  of  Australia,  mostly  different  from 
those  in  Fig.  66.     (Savile  Kent.) 

ate  or  breccia  is  a  cemented  mass  of  coral  pebbles  or  angular  pieces, 
or  is  made  up  of  fragments  of  an  older  coral  rock.  Reef  rock  is 
the  dense  and  solid  mass  formed  by  the  cementing  of  the  finer 
de"bris  which  accumulates  in  quiet  water.  It  is  important  to  notice 
that  even  under  the  microscope  reef  rock  frequently  shows  no 
trace  of  organic  structure,  and  is  a  definite  proof  that  the  absence 
of  such  structure  is  not  a  sufficient  reason  for  denying  the  organic 
origin  of  a  rock.  The  interior  of  growing  masses  which  are  still 
alive  on  the  outside,  and  have  never  been  broken  up,  may  be  so 
crystallized  by  the  action  of  the  sea-water  that  the  organic  struct- 
ure is  obscured  or  destroyed.  On  the  beach  is  formed  a  curious 


CORAL   REEFS 


169 


rock  called  oolite,  which  is  made  up  of  minute  spherules  of  CaCO3 
cemented  into  a  mass  not  unlike  fish-roe  in  appearance.  This  is 
due  to  the  deposition  of  CaCO3  from  solution  around  tiny  grains 


FIG.  68. — Various  forms  of  modern  coral  limestone.     (Savile  Kent.) 

of  calcareous  sand,  until  the  spherules  are  built  up  and  cemented 
together. 

The  growth  of  coral  ceases  when  the  reef  extends  up  to  low- 
water  mark,  but  the  waves  continue  their  work  and  throw  up  debris 
and  build  up  a  platform,  upon  which  they  establish  a  beach  of 


I/O  MARINE   DEPOSITS 

calcareous  sand.  The  latter  may  be  further  piled  up  by  the  winds 
into  dunes  and  solidified  by  the  cementing  action  of  percolating 
rain-water.  According  to  circumstances,  the  new  platform  may  be 
an  extension  of  the  shore  or  an  island  like  the  Florida  Keys. 

Coral  reefs  are  classed  according  to  their  relation  to  the  shore,  and 
are  of  three  kinds,  (i)  Fringing  reefs  are  those  attached  directly 
to  the  land,  though  the  exposed  part  may  be  at  some  distance  out 
from  the  shore  and  separated  from  it  by  a  shallow  channel  with 
coral  bottom.  The  width  of  a  fringing  reef  is  determined  by  the 
slope  of  the  sea-bottom,  being  narrower  on  a  steep  grade,  broader 
on  a  gentle  one.  (2)  Barrier  reefs  are  farther  out  from  shore,  to 
which  the  reef  is  parallel  in  a  general  way,  and  separated  by  a 
broad  and  often  quite  deep  channel.  The  distinction  between  the 
two  kinds  of  reefs  is  not  very  sharply  drawn,  for  the  same  reef  may 
be  fringing  in  parts  of  its  course  and  a  barrier  in  others.  Even 
at  the  present  time  barrier  reefs  are  sometimes  constructed  on  an 
enormous  scale.  A  great  barrier  reef  runs  parallel  to  nearly  the 
whole  north  shore  of  Cuba,  while  the  barrier  reef  of  Australia,  the 
largest  known,  extends,  with  some  breaks,  for  over  1200  miles 
along  the  northeast  coast  of  Australia,  from  which  it  is  distant 
20  to  80  miles  ;  its  breadth  varies  from  10  to  90  miles,  though  but 
little  of  this  width  is  exposed  above  water ;  its  sea-face  is  in  some 
places  more  than  1 800  feet  high  (i.e.  above  the  sea-bottom,  not  the 
surface).  (3)  Atolls  are  coral  islands  of  irregularly  circular  shape, 
which  usually  enclose  a  central  lagoon  and  frequently,  as  in  the 
Pacific,  rise  from  the  profoundest  depths.  The  way  in  which  such 
islands  have  been  built  up  is  still  a  subject  of  much  controversy,  and 
limitations  of  space  forbid  its  discussion  here. 

Dolomitization.  —  A  process  has  been  observed  in  the  closed 
lagoons  of  certain  atolls  which  is  significant  as  throwing  light  upon 
a  very  difficult  problem,  that  of  the  formation  of  dolomite  or 
magnesian  limestone  (see  p.  213).  In  the  closed  lagoon,  shut  off 
entirely  from  the  sea,  the  isolated  body  of  sea-water  becomes  con- 
siderably concentrated  by  evaporation.  All  sea-water  contains 
chloride  of  magnesium  (MgCl),  and  this  percolating  into  the  coral 
rock,  by  double  decomposition  with  CaCO3,  forms  MgCO3.  The 


ORGANIC  ACCUMULATIONS  l"J I 

transformation  takes  place  much  more  readily  when  the  CaCO3  is 
in  the  form  of  aragonite,  as  is  the  case  in  many  shells  and  corals. 
Mollusca.  —  The  ordinary  shell-fish  (Mollusca)  supply  a  very 
large  amount  of  calcareous  material  for  the  formation  of  shallow- 
water  limestones,  especially  in  the  neighbourhood  of  the  coasts. 
The  shells  accumulate  in  great  banks,  frequently,  though  not  always, 
mingled  with  more  or  less  sand  and  mud,  and  when  gathered  below 
the  limit  of  violent  wave  action,  they  are  entire,  embedded  in  finer 
material,  which  is  calcareous  or  not,  according  to  the  nature  of 


FIG.  69. —  Modern  shell  limestone  (Coquina)  from  Florida. 

the  debris  swept  out  from  the  shore.  More  commonly  the  shells 
are  ground  by  the  waves  into  fragments,  making  shell  sand  and 
mud,  which  is  then  cemented  into  a  more  or  less  compact  mass. 
The  coquina  rock  of  Florida  is  an  example  of  a  recently  made 
shell  limestone  (though  it  is  forming  no  longer),  and  among  the 
rocks  of  the  earth's  crust  are  many  immense  limestones  which  were 
accumulated  in  this  way. 

Echinodermata.  —  This  group  of  marine  animals,  which  includes 
the  starfishes,  sea-urchins,  crinoids  or  sea-lilies,  etc.,  is  made 
up  of  forms  which  all  secrete  skeletons  of  calcareous  plates,  and 
which  contribute  largely  to  the  formation  of  marine  limestones. 
At  the  present  day,  however,  they  seldom  build  up  any  extensive 


1/2 


MARINE   DEPOSITS 


masses  unassisted,  but  in  former  ages  of  the  world's  history  they 
did  so  on  a  great  scale.  This  is  particularly  true  of  the  crinoids 
(sea-lilies  or  feather-stars),  which  have  now  become  comparatively 
rare,  but  many  ancient  limestones  are  composed  almost  entirely  of 
their  remains,  and  especially  of  their  hard  and  heavy  stems. 


FlG.  70. — Ancient  limestone  composed  of  various  kinds  of  organisms. 

Limestone  Banks.  —  In  favourable  situations  immense  submarine 
plateaus  or  banks  are  built  up  in  shallow  waters  by  the  accumulated 
remains  of  all  sorts  of  lime-secreting  animals,  corals,  echinoderms, 
molluscs,  worms,  and  Foraminifera.  These  are  well  exemplified  in 
the  Gulf  of  Mexico  and  the  Caribbean  Sea  by  the  great  banks  along 
the  west  coast  of  Florida,  the  Yucatan  bank,  and  the  plateau  which 
extends  from  the  coast  of  Nicaragua  almost  to  Jamaica.  On  these 


LIMESTONE   BANKS 


173 


banks  the  luxuriance  and  fulness  of  life  are  astonishing,  myriads 
of  animals  flourishing  in  the  warm  waters,  and  abundantly  supplied 
with  food  by  the  great  ocean  currents  which  sweep  over  the  banks. 
Innumerable  molluscs,  echinoderms,  and  calcareous  worms  are 
continually  dying  and  adding  their  hard  parts  to  the  sea-floor; 
the  waves  and  tides  sweep  calcareous  sand  and  mud  from  the  coral 
reefs  over  the  flats,  and  all  of  these  masses  are  rapidly  consolidated 
into  rock. 

An  example  of  a  limestone  bank  in  moderately  deep  water  is 
the  Pourtales  plateau,  which  extends  southward  from  the  Florida 


FIG.  71.  — Rock  from  Pourtates  plateau.     (A.  Agassiz.) 

Keys,  and  is  covered  by  90  to  300  fathoms  of  water.  "  The  bot- 
tom is  rocky,  rather  rough,  and  consists  of  a  recent  limestone, 
continually,  though  slowly  increasing  from  the  accumulation  of  the 
calcareous  debris  of  the  numerous  small  corals,  echinoderms,  and 
molluscs,  living  on  its  surface.  These  de"bris  are  consolidated  by 
tubes  of  serpulae ;  the  interstices  are  filled  up  by  Foraminifera  and 
further  smoothed  over  by  nullipores. — The  region  of  this  recent 
limestone  ceases  at  a  depth  varying  from  250  to  350  fathoms, 
and  beyond  it  comes  the  trough  of  the  straits."  (A.  Agassiz.) 
It  is  not  known  how  thick  these  modern  limestone  banks  are, 


1/4  MARINE   DEPOSITS 

but  some  indication  is  given  by  the  raised  terrace  of  modern  lime- 
stone which  occurs  in  northern  Yucatan.  In  this  are  caverns 
which  descend  through  more  than  400  feet  of  such  rock  (without 
reaching  the  bottom),  all  of  which  is  formed  from  the  hard  parts  of 
the  same  species  of  animals  as  still  abound  in  the  neighbouring  seas. 

Chemical  Deposits.  —  It  is  not  known  just  how  important  a  part 
is  played  by  chemical  precipitation  in  the  formation  of  marine 
deposits,  but  probably  a  greater  one  than  has  been  generally  sup- 
posed. Rivers  which  bring  in  quantities  of  CaCO3  in  solution 
may  so  overload  the  sea  with  this  substance  (for  sea-water  will 
dissolve  little  of  it)  that  more  or  less  is  precipitated  in  the  neigh- 
bourhood of  the  land.  On  the  coast  of  Asia  Minor,  for  example, 
are  large  areas  of  sandstone  and  conglomerate,  formed  within  recent 
times  by  the  precipitation  of  CaCO3  in  masses  of  sand  and  gravel, 
binding  them  into  hard  rock.  Similar  examples  are  known  else- 
where. There  is  also  some  reason  to  believe  that  the  decay  of 
marine  animals  evolves  sufficient  carbonate  of  ammonia  to  con- 
vert the  sulphate  of  lime  into  the  carbonate  by  double  decompo- 
sition, and  to  precipitate  the  latter  in  some  quantity. 

3.  Deep-sea  Deposits.  —  The  loo-fathom  line  is  by  Murray 
and  Renard  regarded  as  the  boundary  between  shallow  and  deep 
water,  for  it  generally  marks  the  edge  of  the  continental  shelf, 
from  which  the  bottom  rises  very  gently  to  the  land,  but  slopes 
abruptly  down  to  the  oceanic  depression.  The  great  bulk  of 
the  material  derived  from  the  waste  of  the  land  is  thrown  down 
upon  the  continental  shelf,  within  the  loo-fathom  line,  but  the 
finer  particles  are  carried  farther  out  and  subside  in  deeper  and 
quieter  water.  A  considerable  quantity  of  the  finest  sedimentary 
particles  remains  long  suspended  in  sea-water,  especially  in  the 
cold  water  of  the  polar  seas.  On  the  continental  slopes,  extend- 
ing from  the  loo-fathom  line  to  the  bottom  of  the  great  oceanic 
abysses,  are  laid  down  most  of  the  very  fine  materials  derived  from 
the  land,  which  are  grouped  together  under  the  somewhat  indefi- 
nite term,  mud. 

a.  Terrigenous  Deposits  are  composed  of  materials  chiefly  de- 
rived from  the  shore,  and  occur  in  the  less  profound  depths, 


MUDS   AND   GREEN   SAND  1/5 

1 i )  Blue  Mud.  —  The    materials   of  this  deposit,   which   are 
principally,  though  not  altogether,  derived  from  the  land,  are  very 
heterogeneous.     Quartz  grains  in  an  excessively  fine  state  of  sub- 
division are  very  abundant ;  clay  is  often  a  considerable  ingredient, 
and  then  the  mud  is  plastic  when  wet,  but  it  is  usually  more  earthy 
than  clay-like.    Minute  particles  of  other  terrigenous  minerals,  like 
felspar,  hornblende,  augite,  etc.,  are  common.      CaCO3  is  almost 
always  present,  averaging  7%,  and  in   some  instances  rising  to 
25%  ;  this  is  due  chiefly  to  the  foraminiferal  shells,  both  of  those 
species  which  live  at  the  surface  and  those  which  live  on  the  bottom. 
Siliceous  organisms  are  also  present  to  the  average  amount  of  3  %, 
and  are  principally  diatoms,  radiolarians,  and  spicules  of  sponges. 
Glauconite  is  found  in  nearly  all  the  samples.     The  blue  colour 
of  this  mud  is  due  to  the  sulphide  of  iron,  and  the  organic  matter 
which  prevents  the  oxidation  of  the  sulphide.     Of  the  terrigenous 
deep-sea  deposits  blue  mud  is  the  most  extensively  developed ; 
it  is  estimated  as  covering  14,500,000  square  miles  of  the  sea- 
bottom,  and  surrounds  almost  all  coasts,  and  fills  enclosed  basins 
like  the  Mediterranean  and  even  the  Arctic  Ocean.    The  depths 
at  which  blue  mud  is  found  range  from  125  to  2800  fathoms. 

(2)  Red  Mud  is  a  local  development,  which  occurs  principally 
upon  the  Atlantic  coast  of  Brazil,  and  in  the  Yellow  Sea  of  China. 
Silt  of  this  character,  the  red  colour  of  which  is  due  to  Fe2O3,  is 
brought  down  in  large  quantities  by  the  Amazon  and  the  Orinoco. 
Foraminiferal  shells  are  abundant ;  radiolarians  very  rare. 

(3)  Green  Mud  is  much  the  same  in  character  as  the  blue 
mud,  but  owes  its  green  colour  to  the  higher  percentage  of  glauco- 
nite  which  it  contains. 

(4)  Green  Sand  is  granular  in  appearance,  and  is  made  up 
largely  of  grains  of  glauconite  and  casts  in  that  material  of  the 
interior  of  foraminiferal  shells,  together  with  nearly  50%  of  CaCO3. 
The  green  sands  occur  in  shallower  water  than  the  muds,  and  often 
within  the  loo-fathom  line,  as  in  the  case  of  a  deposit  of  this  kind 
which  is  now  forming  off  the  coast  of  Georgia  and  the  Carolinas. 
The  estimated  area  of  the  green  muds  and  sands  is  1,000,000 
square  miles. 


1/6  MARINE  DEPOSITS 

(5)  Volcanic  Muds.  —  In  the  deeper  water  surrounding  vol- 
canic islands  are  deposits  of  fine  mud  made  from  the  disinte- 
gration of  volcanic  rocks,  mixed  with  considerable  clay,  and  also 
calcareous  matter  derived  from  organisms. 

b.  The  Pelagic  Deposits  are  those,  the  materials  of  which  are 
not  directly  derived  from  the  land,  but  consist  of  matters  carried 
to  the  sea  in  solution  and  extracted  from  the  sea- water  by  the 
agency  of  organisms,  together  with  volcanic  substances  in  a  more 
or  less  advanced  stage  of  decomposition.  Only  rarely  are  terrige- 
nous materials  found  in  these  deposits,  as,  for  example,  off  the 
west  coast  of  Africa,  where  fine  sand,  carried  by  the  wind  from  the 
Sahara,  is  found  in  deep  water,  and  ice-borne  fragments  are  common 
in  high  latitudes.  The  pelagic  deposits  are  found  far  from  land, 
and  to  a  great  extent,  in  the  deepest  oceanic  abysses.  In  these 
profound  depths  the  rate  of  accumulation  is  almost  inconceivably 
slow,  and  the  remains  of  extinct  animals  still  lie  exposed,  or  but 
slightly  covered,  upon  the  ocean  floor. 

(i)  Foraminiferal  Ooze.  —  The  Foraminifera  are  minute  ani- 
mals, each  one  a  tiny  speck  of  jelly,  most  of  which,  in  spite  of  their 
extreme  simplicity  of  structure,  have  the  power  of  secreting  very 
beautiful  and  complex  shells  of  CaCO3.  The  species  which  are 
of  importance  in  this  connection  are  those  which  live  in  infinite 
multitudes  at  the  surface  of  the  ocean,  and  the  most  abundant  at 
the  present  time  are  those  which  belong  to  the  genus  Globigerina, 
whence  this  deposit  is  frequently  called  Globigerina  ooze.  These 
surface  Foraminifera  flourish  best  in  warm  water  and  follow  the 
warm  currents,  often  into  quite  high  latitudes.  Their  shells,  which 
drop  to  the  bottom  as  the  occupants  die,  are  present  in  almost  all 
marine  deposits,  but  near  land  the  terrigenous  materials  prepon- 
derate to  such  a  degree  that  the  Foraminifera  make  up  but  a  slight 
proportion  of  the  deposit.  In  deeper  water,  where  the  wash  from 
the  land  does  not  come,  the  foraminiferal  shells  grow  relatively 
much  more  abundant,  and  when  30%  or  more  of  a  given  sample 
of  the  bottom  consists  of  them,  it  is  classed  as  a  foraminiferal  ooze. 
Other  organisms  which  secrete  calcareous  shells  or  tests  always 
contribute  more  or  less  to  these  oozes  (coral  mud,  echinoderms, 


DISTRIBUTION   OF  BOTTOM   DEPOSITS 


177 


MARINE   DEPOSITS 


molluscs,  nullipores,  etc.).  The  deposit  is  purest  and  most 
typical  in  the  medium  depths  of  the  ocean,  far  from  any  land ;  in 
such  places  the  ooze  may  contain  as  much  as  90%  CaCO3  and  is 
white,  while  nearer  land  the  slight  admixture  of  terrigenous  minerals 
gives  a  pink,  grey,  brown,  or  other  colour  to  the  mass.  Below  the 
depth  of  2500  fathoms  the  proportion  of  CaCO3  becomes  much 
diminished,  owing  to  the  increasing  percentage  of  CO.,  in  the  sea- 
water,  which  attacks  and  dissolves  these  delicate  shells. 

The  foraminiferal  oozes  have  a  vast  geographical  extent,  esti- 
mated at  49,520,000  square  miles,  and  are  especially  developed  in 


FlG.  73.  —  Foraminiferal  ooze.      X  20.     (Agassiz  after  Murray  and  Renard.) 

the  Atlantic,  though  they  are  largely  present  in  all  except  the  polar 
seas,  and  range  in  depth  from  400  to  2900  fathoms. 

(2)  Pteropod  Ooze.  —  The  thin  and  delicate  shells  of  the  mollus- 
can  groups,  known  as  the  pteropods  and  heteropods,  abound  at  the 
surface  of  the  warmer  parts  of  the  ocean,  but  their  dead  shells 
are  found  only  in  depths  of  less  than  2000  fathoms.  In  shallow 
water  (and  even  in  greater  depths  near  land)  the  shells  are  con- 
cealed by  other  kinds  of  material,  but  in  moderate  depths,  far  from 
any  land,  these  shells  sometimes  become  so  frequent  in  the  fo- 
raminiferal ooze  as  to  give  it  a  special  character.  In  its  typical 
development  this  pteropod  ooze  has  been  found  only  in  the  Atlan- 


DIATOM   OOZE  179 

tic,  where  it  covers  some  relatively  small  areas,  in  depths  of  400 
to  1500  fathoms. 

(3)  Radiolarian  Ooze.  —  The  organisms  which  we  have  so  far 
considered  secrete  only  shells  or  tests  of  CaCO3,  but  this  is  not  the 
only  substance  which  is  very  extensively  extracted  from  sea-water 
by  living  beings.  Silica  is  also  dissolved  in  sea-water,  and  various 
organisms  construct  their  tests  of  that  substance.  The  Radiolaria 
are,  like  the  Foraminifera,  a  group  of  microscopic,  unicellular 
animals,  which  secrete  siliceous  tests  of  the  most  exquisite  delicacy 
and  beauty ;  they  live  both  at  the  surface  and  at  the  bottom  of  the 


FIG.  74.  —  Pteropod  ooze.     X  4.     (Agassiz  after  Murray  and  Renard.) 

sea.  Radiolarian  tests  may  be  detected  in  all  sorts  of  marine 
deposits  of  both  deep  and  shallow  water,  but  it  is  only  in  very  pro- 
found depths  that  they  occur  in  quantity  sufficient  to  give  character 
to  the  deposit.  When  20%  or  more  of  a  bottom  deposit  consists 
of  radiolarian  tests,  it  is  called  a  radiolarian  ooze,  but  clay  and 
volcanic  minerals  make  up  most  of  the  materials.  This  ooze  has 
been  found  only  in  the  Pacific  and  Indian  oceans,  where,  it  is  esti- 
mated, it  covers  2,290,000  square  miles  of  the  bottom,  at  depths 
of  2350  to  4475  fathoms. 

(4)  Diatom   Ooze.  —  In  our  study  of  fresh-water  deposits  we 
learned  that  the  siliceous  cases  of  the  microscopic  plants  known  as 


i  So 


MARINE  DEPOSITS 


diatoms  form  considerable  accumulations  in  lakes  and  ponds,  and 
they  also  flourish  abundantly  in  brackish  water  and  in  the  sea. 

Diatoms  are  found  in  many  marine 
deposits,  but  in  relatively  small  quan- 
tities. In  the  Antarctic  Ocean,  how- 
ever, is  an  immense  belt  of  ooze, 
believed  to  cover  10,880,000  square 
miles  and  extending  around  the 
globe,  which  is  largely  made  up  of 
their  frustules.  Besides  the  great 
Antarctic  zone,  an  area  of  some 
40,000  square  miles  is  known  in  the 
North  Pacific.  The  diatom  ooze 
FIG.  75- -Diatom  ooze,  x  150.  entireiy  resembles  the  fresh- water 

( Agassiz  after  Murray  and  Renard.)     , 

deposit,   but   may   be    distinguished 

by  the  presence  of  foraminiferal  and  radiolarian  shells  and  tests. 
The  depths  at  which  this  ooze  is  found  are  from  600  to  2000 
fathoms. 

(5)  Red  Clay.  —  The  profoundest  abysses  of  the  ocean,  far 
from  any  land,  are  covered  with  a  deposit  of  red  clay,  which, 
though  varying  much  in  composition  and  colour,  is  yet  of  a 
quite  uniform  character.  In  these  vast  depths  the  foraminiferal 
shells  are  almost  all  dissolved  by  the  carbonated  sea-water,  but 
some  CaCO3  is  very  generally  present,  averaging  about  6%, 
and  diminishing  in  quantity  as  the  depth  increases.  In  the  less 
profound  abysses  the  red  clay  passes  gradually  into  the  forami- 
niferal oozes,  the  number  of  shells  increasing  until  the  ooze- 
like  character  is  attained.  The  clay  is  derived  from  the  disin- 
tegration and  decay  of  volcanic  substances,  especially  pumice, 
which  floats  upon  water,  often  for  months,  and  drifts  long  dis- 
tances in  the  ocean  currents.  The  greater  part  of  these  volcanic 
materials  is  believed  to  be  derived  from  terrestrial  volcanoes,  but 
the  submarine  vents  doubtless  contribute  largely;  particles  of 
undecomposed  volcanic  minerals  and  glasses  are  also  common. 
In  some  regions  the  clay  is  coloured  chocolate  brown  by  the  oxide 
of  manganese,  and  many  separate  nodules  of  this  substance  are 


ESTUARINE  DEPOSITS  l8l 

found.  The  excessive  slowness  with  which  this  abysmal  deposit  is 
formed,  is  shown  by  the  occurrence,  in  recognizable  quantities,  of 
meteoric  iron,  which  reaches  the  earth  in  the  form  of  meteorites, 
or  "  shooting  stars,"  and  by  the  presence  of  the  remains  of  animals 
which  have  long  been  extinct. 

Of  all  the  oceanic  deposits  the  red  clay  is  much  the  most  widely 
extended,  covering  5 1,500,000  square  miles  of  the  bottom.  Almost 
four- fifths  of  this  vast  area  are  in  the  great  depths  of  the  Pacific  ;  the 
shallower  Atlantic  has  much  more  of  the  foraminiferal  ooze  than 
of  the  red  clay.  The  observed  range  in  depth  is  from  2225  to 
3950  fathoms. 

Comparing  the  marine  deposits  now  accumulating  in  the  sea 
with  the  rocks  of  evidently  marine  origin  which  form  most  of  the 
land,  we  find  that  the  great  bulk  of  these  rocks,  the  sandstones, 
slates,  and  limestones,  are  such  as  are  formed  in  water  of  shallow 
and  moderate  depths,  while  only  rarely  do  we  discover  a  rock,  like 
chalk,  that  implies  really  deep  water. 

VII.   ESTUARINE  DEPOSITS 

An  estuary  is  a  wide  opening  at  the  mouth  of  a  river  into  which 
the  sea  has  penetrated  by  the  depression  of  the  land.  In  such 
bodies  of  water  the  tide  often  scours  with  much  force.  Estuaries 
abound  along  our  Atlantic  coast,  Delaware  and  Chesapeake  bays 
and  the  mouth  of  the  Hudson  being  excellent  examples  of  such. 
The  water  in  them  is  brackish,  and  unfavourable  to  abundant 
aquatic  life,  for  only  a  limited  number  of  marine  animals,  and 
fewer  fresh-water  ones,  flourish  in  brackish  water. 

Estuarine  deposits  are,  in  general,  much  like  those  of  the  sea, 
except  that  they  are  apt  to  be  of  a  finer  grain  for  a  given  depth  of 
water;  muds  are  abundantly  laid  down,  especially  in  the  more 
sheltered  nooks  and  bays,  with  fine  and  coarse  sands  and  gravels 
in  the  more  exposed  situations.  The  sands  are  apt  to  show  a 
confused  stratification  from  the  conflicting  currents  and  eddies  in 
which  they  are  deposited,  but  with  horizontal  layers  formed  at  slack 
water.  Extensive  mud  flats  often  surround  an  estuary,  especially 


1 82  CONSOLIDATION   OF   SEDIMENTS 

if  the  rise  and  fall  of  the  tide  be  great.  On  these  flats,  exposed 
during  low  tide  to  the  sun  and  air,  sun-cracks  are  formed  on  the  dry- 
ing surface,  and  these,  together  with  the  prints  of  raindrops  and  the 
tracks  of  land  animals,  will  be  preserved  when  the  incoming  tide, 
advancing  too  gently  to  scour  the  slightly  hardened  surface  of  the 
flat,  deposits  a  fresh  layer  of  sediment  upon  it.  If  the  estuary  be 
the  opening  of  a  large  river,  considerable  deposits  of  river  sedi- 
ment will,  in  times  of  flood,  be  laid  down  upon  the  other  beds, 
producing  an  alternation  of  fresh  and  brackish  water  beds.  On 
the  coast  of  North  Carolina  somewhat  peculiar  conditions  obtain  ; 
the  low  sand-spits  thrown  up  by  the  waves  enclose  extensive  shal- 
low sounds,  into  which  the  tide  enters  by  only  narrow  openings, 
but  which  have  numerous  streams  flowing  into  them.  At  high 
water  the  incoming  tide  acts  as  a  barrier,  damming  back  the 
river  waters,  checking  their  velocity,  and  causing  them  to  deposit 
their  burdens  of  sediment.  In  course  of  time,  the  sounds  must 
be  silted  up  by  the  rivers. 

For  reasons  that  we  have  already  discussed,  estuaries  are  not 
favourable  to  either  fresh-water  or  marine  organisms,  and  hence 
estuarine  deposits  do  not  contain  any  great  variety  of  remains  of 
either  group.  These  remains  may,  however,  represent  numerous 
individuals,  sufficient  sometimes  to  form  limestone  layers.  Diatoms 
may  also  accumulate  in  great  quantities,  as  in  one  of  the  Baltic 
harbours,  where  they  form  18,000  cubic  feet  of  deposit  annually. 
On  the  other  hand,  estuaries  are  often  favourably  situated  for  the 
reception  and  preservation  of  the  remains  of  land  animals  and 
plants  which  are  swept  into  them  by  streams. 

THE  CONSOLIDATION  OF  SEDIMENTS 

The  processes  of  deposition  upon  the  land  and  beneath  the 
water,  which  we  have  so  far  been  studying,  result,  for  the  most 
part,  only  in  the  bringing  together  of  great  masses  of  loose  and 
incoherent  material.  If  such  masses  are  to  be  properly  com- 
pared with  the  hard  rocks  of  the  earth's  crust,  it  will  be  necessary 
to  show  that  loose  sediments  may  be  consolidated  and  rendered 


CONSOLIDATION   BY   HEAT  183 

hard  and  firm,  like  the  latter.  This  is  not  difficult,  for  we  have 
abundant  evidence  to  prove  that  such  consolidation  actually  does 
take  place,  and  in  a  variety  of  ways. 

(1)  Consolidation  by  Weight  of  Sediment.  —  When  deposited 
on  a  sinking  sea-bottom,  sediments  often  accumulate  in  masses 
of  great  thickness,  and  in  such  cases  the    lower   portions  must 
tend  to  consolidate  from  the  weight  of  the  overlying  masses.     Of 
course,  such  a  process  cannot   be  directty  observed  in  modern 
accumulations,  because  only  the  surface  of  them  is  accessible,  but 
from  the  analogy  of  observed  facts  we  may  safely  infer  that  this 
weight  is  not  without  effect. 

( 2 )  Consolidation  by  Cement.  —  Sediment  is  often  penetrated  by 
percolating  waters,  which  carry  in  solution  various  cementing  sub- 
stances, such  as  SiO2,  CaCO3,  FeCO3,  etc.,  and-  the  deposition  of 
these  materials  in  the  interstices  of  the  loose  sediment  will  bind 
the  particles  into  a  firm  rock.     This  process  we  have  already  had 
occasion  to  observe  in  several  instances,  as  in  the  coral  reefs,  the 
drift-sand  rock  of  Bermuda,  the  modern  sandstones  on  the  coast 
of  Asia  Minor,  and  many  others.     In  all  of  these  cases  the  cement- 
ing substance  is  CaCOj,  but  other  modern  rocks  are  known  in 
which  Fe.,O3,  formed  by  the  oxidation  of  FeCO3,  plays  the  same 
role.     Both  of  these   substances   are   very  common  as  cements 
among  the  ancient  rocks.     A  cementing  effect  may  also  be  pro- 
duced by  reactions  within  the  mass  of  the  sediment  itself,  as  is 
seen  in  the  solidification  of  many  volcanic  ashes  mingled  with 
water  to  form  tuffs. 

(3)  Consolidation  through  Heat.  —  This  maybe  local,  as  in  the 
neighbourhood  of  volcanoes,  or  general  and  due  to  the  internal 
heat  of  the  earth.     For  sediment  to  reach  great  thickness  it  must 
subside,  and  this  subsidence  brings  the  lower  parts  of  the  mass 
deep  down  into  the  crust,  where  they  are  invaded  by  the  earth's 
interior  heat,  and  baked  as  bricks  are  burnt  in  a  kiln.     This  pro- 
cess is  likewise  one  which  cannot  be  directly  observed,  but  the 
effects  of  molten  lava  upon  loose  sediments   may  be  watched, 
and  the   consolidating   power  of  heat  has  been   tested   experi- 
mentally. 


1 84  CONSOLIDATION   OF   SEDIMENTS 

(4)  Consolidation  by  Lateral  Pressure. — This  is  probably  the 
most  widely  acting  and  important  agency  of  consolidation. 
Though  it  acts  so  gradually  and  at  such  depths  that  we  cannot 
see  it  in  operation,  yet  the  inference  is,  none  the  less,  a  safe  one. 
We  shall  see  later  that  very  many  of  the  stratified  rocks  are  no 
longer  in  the  nearly  horizontal  position  in  which  they  were  first  laid 
down,  but  have  been  folded  and  fractured  through  the  operation  of 
great  lateral  pressures.  The  more  intensely  folded  and  compressed 
any  rock  has  been,  the  harder  has  it  become,  not  only  through  the 
mechanical  pressure,  but  by  the  heat  and  the  chemical  changes 
which  such  compression  generates.  In  addition  to  this,  we  know 
from  experiment  that  loose  materials  may  be  consolidated  by  power- 
ful compression.  Certain  exceptional  rocks  of  very  ancient  date 
are  known,  which  are  almost  as  incoherent  as  when  first  accumu- 
lated, but  these  all  retain  their  original  horizontal  position  and  have 
not  been  compressed.  It  must  not  be  supposed,  however,  that 
only  compressed  sediments  have  become  hard,  for  great  areas 
of  scarcely  disturbed  rocks  are  found,  which  are  perfectly  solid 
and  firm;  here  some  other  solidifying  agent  has  been  at  work. 

There  are  certain  other  features  in  which  the  loose  modern 
sediments  differ  from  the  older  and  harder  rocks,  such  as  joints, 
and  cleavage  which  divides  many  rocks  into  thin  plates,  indepen- 
dently of  the  planes  of  stratification.  These  may  be  shown,  how- 
ever, to  be  structures  which  the  rocks  have  acquired  after  their 
formation,  and  therefore  need  not  be  discussed  here. 

The  parallel  is  now  complete  between  the  sediments  which  we 
may  observe  to-day  in  the  process  of  accumulation,  and  the  hard 
stratified  rocks  which  make  up  by  far  the  largest  part  of  the  dry 
land.  For  all  these  ancient  rocks  we  may  find  a  counterpart  in 
sediments  now  forming,  and  we  may  conclude  with  perfect  confi- 
dence that  the  ancient  rocks  were  formed  by  the  same  agencies  as 
the  modern  accumulations.  Every  rock  contains  a  more  or  less 
legible  record  of  its  own  history. 

Summary.  —  The  brief  survey  of  dynamical  geology  which  we 
have  now  taken  has  brought  to  light  many  facts  of  the  highest  sig- 
nificance for  the  interpretation  of  the  earth's  history  as  recorded 


SUMMARY   OF   DYNAMICAL   GEOLOGY  185 

in  the  rocks.  The  materials  of  the  earth's  crust  we  have  found 
to  be  in  a  state  of  ceaseless,  though  very  slow,  circulation,  disin- 
tegrating here,  accumulating  there.  Volcanoes  bring  up  from  the 
interior  of  the  earth  molten  and  fragmental  materials,  which  con- 
solidate into  glassy  or  crystalline  rocks,  or  beds  of  cinders  and 
tuffs.  Earthquakes  shatter  the  rocks  and  shake  down  masses  from 
the  cliffs,  while  changes  of  level  bring  the  sea  over  the  land,  or 
raise  parts  of  the  sea-bottom  into  land  surfaces. 

Everywhere  over  the  land  and  along  the  seacoast  processes  of 
disintegration  are  at  work  upon  the  rocks,  decomposing  them 
chemically  and  breaking  them  up  mechanically.  Rain,  wind,  frost, 
changes  of  temperature,  underground  waters,  rivers,  glaciers,  the 
currents  and  waves  of  the  sea,  all  take  part  in  this  work,  each  in 
its  own  characteristic  way.  The  products  of  this  destruction  are 
transported  by  various  agents,  especially  the  rivers,  to  lakes  and 
the  sea,  though  some  find  a  resting-place,  for  a  longer  or  shorter 
time,  upon  the  land.  Part  of  this  material  is  dissolved  in  water, 
but  the  greater  part  is  mechanically  suspended.  The  suspended 
portion  is  sorted  by  the  power  of  water  and  laid  down  in  sheets 
and  layers  upon  the  beds  of  lakes  or  the  ocean,  forming  stratified 
masses,  while  dissolved  materials  are  largely  extracted  by  the 
agency  of  animals  and  plants,  and  deposited  under  water  as  accu- 
mulations of  calcareous  or  siliceous  sediments.  By  various  pro- 
cesses, these  incoherent  and  loose  masses  are  consolidated ;  they 
may  be  upheaved  to  form  new  land  surfaces,  and  a  new  cycle  of 
destruction  and  reconstruction  will  begin.  These  changes  we 
have  studied  in  order  to  obtain  a  key  for  the  interpretation  of 
the  earth's  history,  which  is  recorded  in  the  rocks,  and  we  have 
found  that  these  records  may  be  so  interpreted  by  the  aid  of  pro- 
cesses which  are  still  at  work.  We  have  yet  much  to  learn,  how- 
ever, before  such  a  systematic  history  can  be  attempted,  and  first 
to  study  the  .ways  in  which  the  rocks  are  actually  arranged  and 
the  disturbances  which  they  have  undergone  :  this  is  structural 
geology,  the  next  division  of  our  subject. 


PART    II 


STRUCTURAL    GEOLOGY 

CHAPTER   X 
THE  ROCKS  OF  THE  EARTH'S   CRUST  —  IGNEOUS   ROCKS 

IN  the  first  section  of  this  book  we  made  a  study  of  the  processes 
and  agencies  which  are  still  at  work  upon  and  within  the  earth, 
tending  to  modify  it  in  one  or  other  particular.  We  there  found 
that  slow  but  ceaseless  cycles  of  change  take  place  on  the  earth's 
surface  and  that  a  continual  circulation  of  material  is  going  on. 

We  have  now  to  take  up  the  second  branch  of  our  subject,  that 
of  structural  geology,  which  deals  with  the  materials  of  the  earth's 
crust,  their  mode  of  occurrence,  and  their  arrangement  into  great 
masses.  Structural  geology  is,  .however,  not  merely  a  descriptive 
study ;  hand  in  hand  with  the  examination  of  the  rock-masses 
must  go  the  attempt  to  explain  their  structure,  and  to  show  how 
they  have  come  to  be  as  we  find  them.  Dynamical  principles 
must  be  continually  called  in  to  interpret  the  facts  of  structure,  and 
many  of  the  principles  of  the  construction,  destruction,  and  recon- 
struction of  rocks  find  their  application  in  the  study  of  structure. 

This  application  cannot,  in  all  cases,  be  made  with  confidence 
and  certainty,  both  because  a  given  structure  may  often  be  re- 
ferred, with,  equal  probability,  to  different  processes,  and  because 
certain  of  the  great  dynamical  agencies  are  so  slow  and  gradual  in 
their  mode  of  operation,  that  no  one  has  ever  been  able  to  observe 
them  at  work.  In  this  latter  class  of  cases  the  agency  must  be 

1 86 


ROCKS  IS/ 

inferred,  not  from  anything  which  we  have  actually  seen  accom- 
plished, but  from  the  traces  which  it  has  left  in  the  structure. 
Under  such  circumstances,  it  need  not  surprise  us  to  find  that 
the  explanation  is  not  always  easy  and  obvious,  but  may  be  very 
problematical,  and  that  great  differences  of  opinion  may  arise  con- 
cerning the  rightful  interpretation  of  a  complex  region. 

Here,  as  in  all  other  provinces  of  geology,  the  historical  stand- 
point is  the  dominant  one.  Our  object  is  to  learn,  not  only  the 
agencies  which  have  produced  the  structures  and  the  way  in  which 
they  operated,  but  also  the  successive  steps  by  which  the  structures 
originated,  the  order  in  which  they  occurred,  and  their  geological 
date.  Thus  they  may  be  coordinated  into  the  great  history  of  the 
earth,  which  it  is  the  main  problem  of  geology  to  construct. 

ROCKS 

The  distinction  between  a  rock  and  a  mineral  is  not  always  an 
easy  one  for  the  beginner  to  grasp,  yet  it  is  essential  that  he 
should  do  so.  A  Rock  is  any  extensive  constituent  of  the  earth's 
crust,  which  may  consist,  though  rarely,  of  a  single  mineral,  but  in 
the  great  majority  of  cases  is  a  mechanical  mixture  of  two  or  more 
minerals.  A  rock  thus  has  seldom  a  definite  chemical  compo- 
sition, or  crystalline  form,  or  homogeneous  internal  structure.  An 
examination  with  the  microscope  almost  always  shows  that  a  rock 
is  an  aggregate  of  distinct  minerals,  which  may  be  all  of  one  kind, 
or  of  many  different  kinds,  in  varying  proportions.  Rocks,  then, 
are  mechanical  mixtures,  and  their  properties  vary  in  proportion 
to  their  various  ingredients,  while  minerals  are  chemical  com- 
pounds (see  p.  9). 

In  ordinary  speech  the  term  rock  is  held  to  imply  a  certain 
degree  of  solidity  and  hardness,  but  in  geological  usage  the 
word  is  not  so  restricted.  Incoherent  masses  of  sand  and  clay 
are  regarded  as  being  rocks,  quite  as  much  as  the  hardest 
granites. 

The  classification  of  rocks  is  a  very  difficult  and  obscure  prob- 
lem, and  would  be  so,  even  were  our  knowledge  much  more  com- 


1 88  IGNEOUS   ROCKS 

plete  and  exhaustive  than  it  is.  There  are,  therefore,  great  diver- 
sities in  the  various  schemes  of  classification  which  have  been 
proposed  and  which  are  still  in  use,  and  all  such  schemes  require 
great  modifications  to  meet  continually  advancing  knowledge. 

Bearing  in  mind  the  principle,  already  emphasized  so  often,  that 
geology  is  primarily  a  historical  study,  the  most  logical  scheme  of 
classification  is  obviously  one  that,  so  far  as  possible,  is  genetic, 
that  is  to  say,  one  which  expresses  in  brief  the  history  and  mode 
of  formation  of  the  rocks.  Other  criteria,  such  as  texture  and 
chemical  and  mineralogical  composition,  must  be  employed  for 
the  minor  subdivisions.  On  this  genetic  principle  we  may  divide 
all  rocks  into  three  primary  classes  or  groups. 

A.  Igneous  Rocks,  those  which  were  melted  and  have  solidified 
by  cooling.     Texture  glassy  or  crystalline. 

B.  Sedimentary    Rocks,    those    which    have    been    laid    down 
(almost  always)  under  water,  by  mechanical,  chemical,  and  organic 
processes.      Rocks  composed  of  more  or  less  rounded  and  worn 
fragments,  seldom  crystalline. 

C.  Metamorphic   Rocks,    those    which    have    been    profoundly 
changed  from  their  original  sedimentary  or  igneous  character,  often 
with  the  formation  of  new  mineral  compounds  in  them.     Texture 
fragmental  or  crystalline. 

IGNEOUS   ROCKS 

We  have  every  reason  to  believe  that  the  igneous  rocks  were  the 
first  to  be  formed,  and  arose,  in  the  first  instance,  from  the  cooling 
of  the  surface  of  the  molten  globe.  In  later  ages  and  at  the 
present  time,  the  igneous  rocks  have  a  much  more  deep-seated 
origin  and  have  either  forced  their  way  to  the  surface,  or  have 
cooled  and  solidified  at  varying  depths  beneath  it.  The  igneous 
rocks  being  thus  the  primitive  ones,  all  the  others  have  been 
derived,  either  directly  or  indirectly,  from  them.  The  products  of 
the  chemical  disintegration  or  mechanical  abrasion  of  the  igneous 
rocks  have  furnished  the  materials  out  of  which  the  sedimentary 
rocks  were  formed,  at  least  in  the  first  instance. 


TEXTURES  189 

The  igneous  rocks  are  massive,  as  distinguished  from  stratified, 
and  though  sometimes  presenting  a  deceptive  appearance  of  strati- 
fication, may  always,  with  a  little  care,  be  readily  distinguished 
from  the  truly  stratified  rocks.  The  term  massive  is,  indeed, 
frequently  used  for  these  rocks,  in  the  same  sense  as  igneous,  and 
eruptive  rocks  is  another  term,  meaning  the  same  thing,  though 
eruptive  is  also  employed  in  a  more  restricted  sense. 

Characteristic  differences  appear  between  those  igneous  masses 
which  have  solidified  deep  within  the  earth  and  have  been  brought 
to  light  only  by  the  denudation  and  removal  of  the  overlying 
rock-masses,  and  those  which  have  cooled  at  or  near  the  sur- 
face of  the  ground.  The  former  are  called  plutonic  (or  intru- 
sive) and  the  latter  volcanic  (or  extrusive).  Between  the  two 
may  be  found  every  form  of  transition,  and  the  terms  volcanic 
and  plutonic  are  now  employed  for  description  rather  than  for 
classification. 

The  texture  of  an  igneous  rock  means  the  size,  shape,  and  mode 
of  aggregation  of  its  constituent  minerals.  Texture  is  a  very  im- 
portant means  of  determining  the  circumstances  under  which  the 
rock  was  formed,  and  hence  great  attention  is  paid  to  it.  Since 
texture  responds  so  accurately  to  the  circumstances  of  solidification, 
^ate  of  cooling,  pressure,  etc.,  all  the  varieties  shade  into  one 
another  by  imperceptible  gradations  and  form  a  continuous  series. 
Nevertheless,  it  is  necessary  to  distinguish  .and  name  the  more 
important  kinds. 

Among  the  igneous  rocks  are  found  four  principal  types  of 
texture,  with  several  minor  varieties. 

i.  Glassy,  -—  _Here  the  rock  is  a  glass  or  slag,  without  distinct 
minerals  in  it,  though  the  incipient  stages  of  crystallization,  in  the 
form  of  globules  and  hair-h'k^  rods,  are  often  observable  with  the 
microscope.  When  the  glass  or  slag  is  made  frothy  by  the  bubbles 
of  escaping  steam  and  gas,  the  texture  is  said  to  be  vesicular, 
scoriaceous,  or  pumiceous,  au^rding  to  the  abundance  of  the 
bubbles.  These  are  varietiesJ^Pthe  glassy  texture,  though  other 
kinds  may  also  be  vesicular,  ^^esicular  rock  in  which  the  steam- 
holes  have  been  filled  up  by  the  subsequent  deposition  of  some 


IGNEOUS   ROCKS 

mineral,  is  called  amygdaloidal,  a  term  derived  from  the  Greek 
word  for  almond. 

2.  The   Compact  (or  Felsitic)  texture  is  characterized  by  the 
formation  of  exceedingly  minute  crystals,  too  small  to  be  seen  by 
the  unassisted  eye,   giving  the  rock  a  homogeneous,   but   stony 
and  not  glassy  appearance.     If  the  crystals  are  too  minute  to  be 
identified  even  by  the  aid  of  the  microscope,  it  is  said  to  be  cryp- 
tocrystalline,    and   when   such    identification   can   be  made,  it  is 
called  microcry stalling. 

3.  Porphyritic. —  In   rocks  of  this  texture   are   large,   isolated 
crystals,  called  phenocrysts,  embedded  in  a  ground  mass,  which  may 
be  glassy  or  made  up  of  minute  crystals.     The  phenocrysts  may 
have  sharp  edges  and  well-formed  faces,  or  they  may  have  irregu- 
lar and  corroded  surfaces.      The  porphyritic  texture  indicates  two 
distinct  phases  of  crystallization.     The  first  is  the  formation  of  the 
phenocrysts,   which   remain    suspended    in   the   molten   mass,   or 
magma,  and  are  often  corroded  and  partially  redissolved  by  it. 
These  crystals  are  said  to  be  Qiintratelluric  origin,  because  formed 
before  the  eruption  of  the  lava,  and  such  crystals  are  showered  out 
of  certain  active  volcanoes  at  the  present  time.      Stromboli  (see 
p.  36),  for  example,  ejects  quantities  of  large  and  perfect  augite 
crystals.     There  is  reason  to  believe,  however,  that  not  all  pheno,- 
crysts  are  thus  intratelluric,  but  that  the  first  phase  of  crystallization 
sometimes  takes  place  after  the  ejection  of  the  molten  mass.     The 
second  phase  consists  in  the  formation  of  the  ground  mass,  which 
may  be  glassy,  minutely  crystalline,  or  both. 

4.  Granitoid.  —  In  this  texture  the  rock  is  wholly  crystalline, 
without  ground  mass  or  interstitial  paste.     The  component  grains, 
which  may  be  fine  or  very  coarse,  are  of  quite  uniform  size,  and 
as  the  crystals  have  interfered  with  one  another  in  the  process  of 
formation,  they  have  rarely  acquired  their  proper  crystalline  shape. 
Such  grains  are  said  to  be  allotriomorphic. 

An  additional  texture  which  should  be  mentioned  is  \hzf rag- 
mental.  This  is  represented  by  the  accumulations  of  the  frag- 
mental  products  ejected  by  volcanoes  (see  p.  51),  agglomerates, 
bombs,  lapilli,  ashes,  etc.  Many  such  materials  accumulate  in 


TEXTURES  191 

bodies  of  water  and  are  there  sorted  and  stratified  and,  it  may  be, 
mingled  with  more  or  less  sand  and  mud  and  other  truly  sedi- 
mentary material.  Rocks  formed  in  this  manner  partake  of  the 
nature  of  both  the  igneous  and  sedimentary  classes,  and  may  be 
referred  to  either  group,  or  regarded  as  a-  series  intermediate 
between  the  other  two  and  in  a  measure  connecting  them.  These 
rocks  will  here  be  treated  as  a  special  subdivision,  under  the  name 
of  pyroclastic  rocks. 

In  our  studies  of  the  products  of  modern  volcanoes,  we  saw  that 
the  same  molten  mass  will  give  rise  to  rocks  of  very  different 
appearance  in  its  different  parts,  according  to  the  circumstances 
of  rapidity  of  cooling,  pressure,  etc.  We  may  now  express  this  in 
somewhat  more  general  form  and  say  that  the  texture  of  an  igne- 
ous rock  is  determined  by  the  several  factors  which  affect  the 
molten  mass  during  consolidation.  Of  such  factors  may  be  men- 
tioned the  chemical  composition,  temperature,  rate  of  cooling, 
degree  of  pressure,  and  the  quantity  present  of  dissolved  vapours 
and  gases,  which  are  called  mineralizers.  In  one  and  the  same 
continuous  mass  of  rock  we  also  find  great  differences  of  minera- 
logical  composition,  a  process  of  segregation  taking  place  in  the 
molten  magma.  When  this  occurs,  it  is  the  general  rule  that  the 
mass  becomes  more  basic  toward  the  periphery. 

Chemical  composition  determines  the  fusibility  of  a  rock  at  a 
given  temperature.  The  least  fusible  rocks  are,  on  the  one  hand, 
those  which  contain  large  quantities  of  silica,  60  to  75%  (acid 
rocks),  and,  on  the  other,  those  which  contain  less  than  40%  of 
silica  (ultrabasic  rocks).  The  most  fusible  rocks  are  those  with 
an  intermediate  percentage  of  silica  (basic  rocks),  and  among  these 
the  fusibility  increases,  as  the  percentage  of  silica  diminishes,  until 
the  lower  limit  is  reached.  The  effect  of  chemical  composition 
upon  texture  is  seen  in  the  rapidity  with  which  the  less  fusible 
rocks  chill  and  stiffen,  and  therefore  the  greater  frequency  with 
which  they  form  glasses. 

Chemical  composition  is,  however,  important  in  this  connection 
chiefly  through  its  effect  upon  the  rate  of  solidification.  We  have 
already  learned  (p.  12)  that  solidification  very  generally  takes 


1 92  IGNEOUS   ROCKS 

place  by  a  process  of  crystallization,  and  this  requires  time.  Hence, 
very  rapid  cooling  results  in  a  glass,  but  the  microscope  reveals 
the  incipient  stages  of  crystallization  in  many  of  even  the  glassy 
rocks.  A  somewhat  slower  rate  of  solidification  produces  a 
cryptocrystalline  rock,  and  successively  slower  rates  bring  about 
the  porphyritic,  microcrystalline,  and  granitoid  textures.  Large 
crystals  form  slowly,  and  other  things  being  equal,  the  larger 
the  component  crystals  of  a  rock,  the  more  slowly  has  it 
consolidated. 

Pressure  is  of  importance  in  preventing  the  rapid  escape  of  the 
vapours  and  gases  contained  in  the  molten  mass,  and  hence  frothy, 
scoriaceous,  and  vesicular  textures  cannot  be  produced  under  high 
pressures.  Pressure  is  also  believed  to  be  necessary  for  the  forma- 
tion of  many  phenocrysts  in  porphyritic  rocks. 

The  mineralizers,  such  as  steam,  hydrochloric  acid,  and  other 
vapours,  determine  the  crystallization  of  many  minerals,  which 
refuse  to  crystallize  in  the  absence  of  such  vapours.  Variations  in 
the  quantity  of  mineralizers  present  in  different  parts  of  the  same 
mass,  occasion  corresponding  differences  in  the  local  textures. 
The  well-known  Obsidian  Cliff,  in  the  Yellowstone  National  Park, 
is  formed  by  a  great  sheet  of  igneous  rock,  made  up  of  alternating 
layers  of  glassy  and  microcrystalline  rock,  a  difference  which  is 
referred  to  varying  proportions  of  mineralizers  present  in  different 
parts  of  the  molten  mass. 

It  must  not  be  supposed  that  a  molten  magma  consists  merely 
of  a  number  of  fused  minerals,  mechanically  mixed  together  and 
having  no  effect  upon  one  another.  If  such  were  the  case,  the 
minerals  in  cooling  should  all  crystallize  in  the  order  of  their  fusi- 
bility, the  least  fusible  forming  first,  and  the  most  fusible  last. 
This  is  not  what  we  find,  and  many  facts  which  cannot  be  discussed 
here  have  led  petrographers  to  the  belief  that  a  molten  magma 
is  a  solution  of  certain  compounds  in  others,  and  that  crystalliza- 
tion occurs  in  the  order  of  solubility,  as  the  point  of  saturation  for 
'particular  compounds  is  successively  reached  by  the  cooling  mass. 

Similar  phenomena  may  be  observed  among  the  metals.  If 
strips  of  copper  be  thrown  into  a  vessel  of  melted  tin,  the  latter 


CRYSTALLIZATION   OF  MAGMA  193 

will  dissolve  the  copper  at  a  temperature  far  below  that  at  which 
the  copper  would  melt  alone. 

In  a  rock  magma  the  crystallization  of  the  more  and  more 
soluble  minerals  will  proceed  regularly,  provided  the  pressure  and 
rate  of  cooling  continue  constant.  As  these  conditions  are,  how- 
ever, subject  to  variation,  it  frequently  happens  that  the  more 
soluble  minerals  begin  to  crystallize  before  the  less  soluble  have 
all  been  formed,  and  thus  the  periods  of  formation  of  two  or  more 
kinds  of  minerals  partly  overlap. 

Usually,  the  mode  of  formation  of  the  different  kinds  of  min- 
erals in  a  solidifying  mass  is  as  follows.  First  to  form  are  apatite, 
the  metallic  oxides  (magnetite,  ilmenite)  and  sulphides  (pyrites), 
zircon,  and  titanite.  "  Next  come  the  ferro-magnesian  silicates, 
olivine,  biotite,  the  pyroxenes,  and  hornblende.  Next  follow  the 
felspars  and  felspathoids,  nepheline  and  leucite,  but  their  period 
often  laps  well  back  into  that  of  the  ferro-magnesian  group.  Last 
of  all,  if  excess  of  silica  remains,  it  yields  quartz.  In  the  variations 
of  pressure  and  temperature,  it  may  and  often  does  happen  that 
crystals  are  again  redissolved,  or  resorbed,  as  it  is  called,  and  it 
may  also  happen  that  after  one  series  of  minerals,  usually  of 
large  size  and  intratelluric  origin,  have  formed,  the  series  is  again 
repeated  on  a  small  scale,  as  far  back  as  the  ferro-magnesian 
silicates.  Minerals  of  a  so-called  second  generation  thus  result, 
but  they  are  always  much  smaller  than  the  phenocrysts  and  are 
characteristic  of  the  ground  mass. 

"  It  results  from  what  has  been  said  that  the  residual  magma  is 
increasingly  siliceous  up  to  the  final  consolidation,  for  the  earliest 
crystallizations  are  largely  pure  oxides.  It  is  also  a  striking  fact 
that  the  least  fusible  minerals,  the  felspars  and  quartz,  are  the 
last  to  crystallize."  (Kemp.) 

A  very  considerable  number  of  minerals  are  found  in  the 
igneous  rocks,  but  comparatively  few  in  any  large  quantity.  It 
thus  becomes  necessary  to  distinguish  between  the  essential  min- 
erals of  a  rock  and  the  accessory  ones.  The  essential  minerals  are 
those  the  presence  of  which  is  necessary  to  the  formation  of  a 
given  kind  of  rock,  while  the  accessory  minerals  are  those  which 


194  IGNEOUS   ROCKS 

occur  in  small  quantities  and  which  may  be  present  or  absent, 
without  affecting  the  character  of  the  rock.  The  distinction  is 
necessary  and  useful,  but  is  sometimes  arbitrary. 

Another  necessary  distinction  is  that  between  original  and 
secondary  minerals.  Original  minerals  were  formed  with  or  be- 
fore the  rock  of  which  they  are  constituents,  and  secondary 
minerals  are  produced  by  the  alteration  or  reconstruction  of  the 
original  ones. 

With  comparatively  few  exceptions,  the  igneous  rocks  are  made 
up  of  some  felspar  or  felspathoid,  together  with  one  or  more  of 
the  pyroxenes,  amphiboles,  micas,  or  qujrtz.  Magnetite  is  also 
very  common. 

What  was  said  above  with  regard  to  the  difficulty  of  classifying 
rocks,  applies  more  especially  to  the  igneous  group,  because  of 
the  way  in  which  the  various  kinds  shade  into  one  another,  since 
even  the  same  molten  mass  may  differentiate  into  several  species, 
showing  not  only  differences  of  texture,  but  marked  changes  of 
chemical  and  mineralogical  composition.  In  an  elementary  work, 
like  the  present,  only  a  meagre  outline  of  the  subject  can  be 
attempted,  for  the  microscopic  study  of  rocks,  or  petrography, 
has  now  become  an  independent  science  of  great  scope  and  inter- 
est, and  cannot  be  compressed  into  a  few  pages. 

The  classification  of  the  igneous  rocks  now  most  generally 
adopted,  is  made  upon  a  threefold  method,  according  to  texture, 
and  chemical  and  mineralogical  composition.  In  the  following 
table  (modified  from  Kemp's)  the  textures  are  given  in  vertical 
order,  while  transversely  the  arrangement  is  mineralogical,  chiefly 
in  accordance  with  the  principal  felspar.  In  this  manner  the 
acidic  rocks  come  at  the  left  side  of  the  table  and  the  basic  at  the 
right  side.  The  percentages  of  silica  are  given  on  a  lower  line  of 
the  table. 


CLASSIFICATION 


195 


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Leucite 


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196  IGNEOUS   ROCKS 

The  acidic  rocks  are  so  called  because  they  are  rich  in  silica 
(which  is  an  acid),  but  they  have  only  small  quantities  of  lime, 
magnesia,  and  iron ;  hence  they  are  very  infusible,  of  low  specific 
gravity,  and  generally  of  light  colours.  The  basic  rocks,  thus 
named  because  of  the  predominance  of  the  bases,  have  much 
smaller  percentages  of  silica  and  higher  ones  of  lime,  magnesia, 
and  iron ;  the  latter  substances  act  as  fluxes,  making  the  basic 
rocks  much  more  fusible,  as  well  as  giving  them  a  higher  specific 
gravity  and  darker  colour.  The  distinction  between  acidic  and 
basic  rocks  cannot  be  very  sharply  drawn,  because  the  two  kinds 
are  connected  by  every  variety  of  intermediate  gradation.  The 
same  is  true,  however,  of  all  the  divisions  given  in  the  table, 
which  is  apt  to  produce  a  false  impression  of  sharply  distinguished 
groups  of  rocks,  such  'as  do  not  occur  in  nature. 

As  a  general  rule,  the  glassy  and  porphyritic  textures  charac- 
terize those  rocks  which  have  solidified  at  the  surface  of  the 
•ground,  or  not  very  far  below  it,  while  the  granitoid  types  have 
cooled  slowly  and  at  great  depths ;  but  there  are  exceptions  to 
both  statements.  Between  the  glassy  and  porphyritic  textures  at 
one  end  of  the  series  and  the  granitoid  at  the  other,  comes  the 
felsitic,  which  represents  an  intermediate  rate  of  cooling  and 
intermediate  depths  within  the  earth  as  the  place  of  solidification. 

The  division  of  the  igneous  rocks  into  families  is  made  prima- 
rily in  accordance  with  the  mineralogical  composition,  with  sub- 
divisions according  to  texture.  This  method  gives  us  five  principal 
groups-.  '-••  " 

I.   THE  GRANITE  FAMILY 

The  molten  magma,  which  on  solidification  gives  rise  to  the  rocks 
of  this  group,  is  very  rich  in  silica  (65  to  75%)  and  has  from  10 
to  15%  of  alumina;  the  Quantity  pf  alkalies  (Na  and  K)  is  rela- 
tively large  (6  to  8%),  and  there  are  small  amounts  of  iron 

.oxides  (2  to  4%),  magnesia  (i  to  2%),  and  lime  (i  to  4%). 
In  the  process  of  consolidation  the  principal  minerals  formed  are 

'orthoclase  and  quartz,  with  smaller  amounts  of  oligoclase,  iron 
oxide,  and  of  the  ferro-magnesian  minerals,  biotite  or  hornblende. 


GRANITE    FAMILY  197 

Differences  of  texture,  produced  in  the  manner  already  described, 
give  rise  to  rocks  of  totally  different  appearance,  which  it  is  diffi- 
cult to  imagine  are  of  similar  or  identical  composition. 

Obsidian  is  a  volcanic  glass,  which  is  usually  black  or  dark 
brown  or  green  (but  sometimes  blue,  red,  or  yellow).  It  breaks 
with  a  shell-like  fracture,  and  in  very  thin  pieces  is  translucent. 
The  microscope  shows  that  its  dark  colour  and  opacity  are  due  to 
the  quantity  of  minute  "  crystallites,"  the  incipient  stages  of  crys- 
tals, which  are  present  in  great  numbers.  The  name  obsidian  is 
used  for  the  various  kinds  of  volcanic  glass  in  which  the  percent- 
age of  water  is  small,  and  so  for  exact  description  a  prefix  is 
necessary,  such  as  rhyolite  obsidian,  andesite  obsidian.  Though 
the  glasses  are  of  varying  composition,  by  far  the  greater  number 
of  them  belong  to  the  granite  family.  When  the  glass  is  divided 
by  concentric  cracks,  due  to  shrinkage  on  cooling,  so  as  to  form 
onion-like  spherules,  it  is  called  Perlite. 

Pitchstone  has  much  the  same  appearance  as  obsidian,  but 
contains  from  5  to  10%  of  water. 

Pumice  is  a  glass  blown  up  by  the  bubbles  of  escaping  steam 
and  other  vapours  into  a  rock  froth,  so  light  that  it  will  float  upon 
water.  A  very  similar  substance  is  produced  when  a  jet  of  steam 
is  blown  through  the  melted  slag  from  an  iron  furnace. 

It  not  infrequently  happens  that,  in  course  of  time,  the  volcanic 
rocks  become  devitrijud,  losing  their  glassy  texture  and  assuming 
a  stony  one.  The  homogeneous"  rock  becomes  converted  into  a 
mass  of  extremely  minute  crystals  of  quartz  and  felspar,  and 
the  original  glassy  texture  is  then  shown  only  by  the  lines  of 
flow,  or  by  the  perlitic  character,  which  are  not  affected  by  the 
change. 

Rhyolite  ordinarily  occurs  as  the  lava  outflow  of  a  granitic 
magma,  cooled  rapidly,  but  yet  more  slowly  than  obsidian.  The 
texture  is  porphyritic,  the  phenocrysts  being  chiefly  quartz,  and 
the  glassy  form  of  orthoclase  known  as  sanidine,  while  the  ferro- 
magnesian  minerals  are  present  in  very  much  smaller  quantities, 
and  of  these  the  commonest  is  biotite.  The  phenocrysts  are 
embedded  in  a  ground  mass  of  minute  felspar  crystals  and  a 


198  IGNEOUS   ROCKS 

varying  proportion  of  glass.  Other  names  used  for  rhyolite  are 
liparite  and  quartz  tracJiyte.  The  rhyolites  are  exceedingly  com- 
mon in  the  western  part  of  the  United  States.  The  Felsites  are 
very  dense,  fine-grained,  and  light-coloured  rocks,  in  which  pheno- 
crysts  are  absent  or  scanty ;  they  are  ancient  rocks  which  have 
been  formed  in  different  ways,  by  the  devitrification  of  obsidians 
and  rhyolites,  by  the  recrystallization  of  tuffs,  and  by  original  cool- 
ing from  fusion. 

Quartz  Porphyry  shades  imperceptibly  into  rhyolite  or  felsite 
on  the  one  hand,  and  into  granite  on  the  other ;  it  is  made  up 
of  phenocrysts  of  quartz,  or  of  quartz  and  orthoclase,  in  a  finely 
crystalline  ground  mass  of  the  same  minerals.  If  the  phenocrysts 
are  all  of  orthoclase,  the  rock  is  called  a  felspar  porphyry.  The 
difference  from  rhyolite  consists  in  the  greater  compactness  and 
density  of  the  rock  and  the  absence  of  glassy  ground  mass. 

Granite.  —  The  granites  are  thoroughly  crystalline  rocks,  of 
typically  granitoid  texture,  to  which  they  have  given  the  name, 
and  without  any  ground  mass.  The  grains  have  not  their  proper 
crystalline  shape,  the  separate  minerals  interfering  with  each  other 
in  the  process  of  crystallization.  The  characteristic  minerals  are 
quartz,  orthoclase,  some  acid  plagioclase,  muscovite,  and  biotite ; 
magnetite  and  apatite  are  always  present,  though  in  small  quan- 
tities. The  variations  in  granite  are  principally  in  the  ferro- 
magnesian  minerals.  Thus  we  have  muscovite  granite,  with  white 
mica  only  ;  granitite,  with  biotite  only  ;  hornblende  granite,  the 
hornblende  replacing  the  mica,  or  in  addition  to  biotite ;  augite 
granite,  with  augite  and  biotite.  Those  in  which  .the  percentage 
of  soda  is  high  are  called  soda-granites.  When  the  dark  silicates 
and  mica  are  all  absent,  the  rock  is  called  a  binary  granite. 

The  colour  of  granite  is  dark  or  light  in  accordance  with  the 
proportion  of  dark  silicates  present,  while  the  shades  of  the  felspar 
determine  whether  the  rock  shall  be  red,  pink,  or  white.  The 
texture  of  granite  varies  from  fine  to  very  coarse,  and  in  some 
cases  there  is  found  a  ground  mass  of  ordinary  granite,  in  which 
are  embedded  very  large  crystals  of  orthoclase ;  this  is  sometimes 
called  a  granite  porphyry. 


SYENITE   FAMILY  1 99 

II.   THE  SYENITE  FAMILY 

In  this  family  the  magma  much  resembles  that  of  the  granite 
group,  except  that  the  quantity  of  silica  is  less  (55  to  65  %);  hence 
it  is  nearly  or  quite  taken  up  in  the  formation  of  silicates,  leaving 
little  or  none  to  crystallize  out  separately  as  quartz,  and  orthoclase 
is  thus  the  chief  mineral.  The  two  families  are  connected  by 
many  transitional  rocks. 

Syenite  obsidian  is  indistinguishable,  except  by  chemical  analy- 
sis, from  the  glasses  of  the  preceding  family,  but  it  is  much  less 
common. 

Trachyte  is  a  volcanic  rock,  consisting  of  phenocrysts  of  sani- 
dine  in  a  ground  mass  of  minute  felspar  crystals,  but  having  little 
or  no  glass,  together  with  more  or  less  biotite,  amphibole,  or 
pyroxene,  according  to  which  we  get  the  varieties  mica,  amphi- 
bole, or  pyroxene  trachyte.  In  America  the  trachytes  are  very 
much  less  abundant  than  the  rhyolites. 

Phonolite  differs  from  trachyte  in  the  higher  percentage  of 
soda  which  it  contains,  and  in  the  presence  of  the  felspathoid 
nepheline  or  leucite,  or  both.  The  name  is  derived  from  the 
ringing  sound  which  thin  plates  of  the  rock  give  out  when  struck 
with  a  hammer.  Phonolites  are  quite  rare  rocks,  and  in  this 
country  the  best-known  locality  for  them  is  the  Black  Hills  region 
of  South  Dakota. 

Syenite  is  a  thoroughly  crystalline  rock,  without  ground  mass, 
and  much  resembling  granite  in  appearance,  but  having  no  quartz. 
It  is  composed  typically  of  orthoclase  and  hornblende,  with  plagio- 
clase,  apatite,  and  magnetite  as  accessories.  When  the  hornblende 
is  replaced  by  biotite,  the  rock  is  called  mica  syenite,  and  when  by 
augite,  augite  syenite.  The  name  syenite  is  sometimes  given  to 
the  rock  we  have  called  "hornblende  granite  "  (p.  198). 

Nepheline  Syenite  is  marked  by  the  presence  of  nepheline,  and 
bears  the  same  relation  to  phonolite  as  syenite  does  to  trachyte, 
being  the  granitoid  crystallization  of  the  same  magma. 

The  syenites  occur  just  as  do  the  granites,  but  are  not  nearly 
so  frequent. 


2OO  IGNEOUS   ROCKS 


III.  THE  DIORITE  FAMILY 

The  magma  of  these  rocks  has  about  the  same  silica  percent- 
ages (55  to  65  %)  as  have  the  syenites,  but  the  quantity  of  alkalies 
is  less,  while  that  of  the  lime  and  magnesia  is  greater.  Hence 
orthoclase  is  absent  or  much  less  important,  and  the  principal 
mineral  is  a  soda-lime  felspar.  The  textures  display  the  usual 
variety  from  glassy  to  granitoid. 

The  glasses  of  this  family  (andesite  obsidian)  can  be  distin- 
guished from  those  of  the  preceding  groups  only  by  chemical 
analysis,  but  they  are  rare. 

Andesites  are  dark-coloured  lavas  of  porphyritic  or  compact 
texture,  composed  of  a  glassy  plagioclase  felspar  and  some  ferro- 
magnesian  mineral,  embedded  in  a  ground  mass  of  felspar  needles 
and  glass.  In  accordance  with  the  nature  of  the  predominant 
ferro-magnesian  mineral,  we  have  hornblende  andesite,  biotite  ande- 
site, and  several  varieties  of  pyroxene  andesite.  These  rocks  are 
very  common  in  the  western  United  States  and  along  the  Pacific 
coast  of  both  North  and  South  America ;  they  are  named  from 
the  Andes. 

The  Dacites  differ  from  the  andesites  in  having  quartz,  and 
therefore  a  higher  percentage  of  silica. 

The  Diorites  are  the  plutonic  equivalents  of  the  andesites  and 
dacites,  having  granitoid  texture,  but  they  are  usually  of  much 
finer  grain  than  the  granites  and  syenites.  The  ferro-magnesian 
mineral  is  usually  green  hornblende,  but  augite  and  other  pyrox- 
enes and  biotite  occur  in  the  different  varieties.  Most  diorites 
have  a  little  quartz ;  but  when  this  mineral  becomes  abundant,  it 
gives  a  quartz  diorite,  which  is  related  to  the  dacites  as  the  typical 
diorite  is  to  the  andesites.  A  common  name  for  the  diorites  is 
greenstone. 

IV.  THE  BASALT  FAMILY 

In  the  magmas  of  this  series  the  percentage  of  silica  is  much 
less  than  in  the  preceding  groups  (40  to  55  %),  and  the  quantity 


BASALT  FAMILY  2OI 

of  alkalies  is  small,  while  that  of  iron,  magnesia,  and  lime  is  much 
greater.  They  are  heavy,  dark-coloured  rocks,  which  generally 
weather  red.  from  the  oxidation  of  the  FeO  which  they  contain. 
The  principal  minerals  are  a  plagioclase  felspar,  rich  in  lime 
(labradorite  or  anorthite),  some  kind  of  pyroxene,  magnetite, 
and  frequently  olivine.  There  is  a  wide  range  of  mineralogical 
composition  and  many  varieties  of  rock  occur  in  this  family,  but 
often  these  can  be  distinguished  from  one  another  only  by  the  aid 
of  the  microscope. 

Tachylyte  is  a  basaltic  glass,  which  is  not  at  all  common. 

Basalt  is  a  name  of  wide  application  covering  many  varieties, 
which,  however,  can  seldom  be  distinguished  by  the  unassisted 
eye.  They  are  very  common  volcanic  rocks,  and  most  of  the 
active  volcanoes  of  the  present  day  extrude  basaltic  lavas.  In 
texture  the  basalts  are  ordinarily  porphyritic,  but  they  may  be 
without  phenocrysts,  and  consist  of  a  finely  crystalline  mass.  The 
ground  mass  is  made  up  of  tiny  crystals,  mingled  with  a  dark 
glass. 

The  basalts  are  closely  related  to  the  andesites  and  connected 
with  them  by  a  number  of  transitional  forms,  but  in  the  andesites 
the  phenocrysts  are  principally  felspars,  which  is  not  the  case  in 
the  basalts.  Those  basalts  which  contain  olivine  in  notable  quan- 
tities are  called  olivine  basalt;  while  those  in  which  the  felspar  is 
replaced  by  leucite  or  nepheline  are  called  leucite  and  nepheline 
basalt,  respectively.  Several  other  named  varieties  occur,  but, 
for  the  most  part,  they  require  the  aid  of  the  microscope  for 
their  identification.  A  rare  variety,  found  in  New  Mexico,  Cali- 
fornia, and  elsewhere,  contains  phenocrysts  of  quartz. 

Trap  is  a  useful  field  name  for  various  sorts  of  dark,  granular 
rocks,  which  cannot  readily  be  distinguished  by  inspection.  The 
term  is  often  applied  to  diorite  and  especially  to  diabase. 

Dolerite  is  a  coarsely  crystalline  basaltic  rock,  which  is  either 
porphyritic  or  granitoid  in  texture. 

Diabase  is  a  rock  of  peculiar  texture ;  the  felspar  crystals  are 
long,  narrow,  and  lath-shaped,  and  contain  the  dark  minerals  in 
their  interstices.  The  trap  rocks  of  the  Palisades  of  the  Hudson, 


202  IGNEOUS   ROCKS 

and  many  localities  in  the  Connecticut  valley,  New  Jersey,  Mary- 
land, Virginia,  and  North  Carolina,  are  diabase. 

Gabbro  is  a  term  which  is  now  used  comprehensively  to  include 
the  coarse-grained,  plutonic  phases  of  the  various  basaltic  rocks, 
which  are  typically  composed  of  plagioclase  and  pyroxene.  Ottvine 
gabbro  and  hornblende  gabbro  are  names  that  explain  themselves. 
No  rife,  or  hypers  thene  gabbro,  contains  orthorhombic  pyroxene. 
Anorthosite  is  nearly  pure  labradorite  in  large  crystals,  with  little 
or  no  pyroxene  :  great  masses  of  it  occur  in  Canada  and  the 
Adirondack  Mountains  of  New  York. 

NOTE.  —  Since  the  foregoing  paragraphs  were  written,  Professor  Pirsson  has 
described  a  group  of  basaltic  rocks  which  have  large,  transparent  phenocrysts 
of  analcite  with  pyroxene,  olivine,  and  other  dark  silicates  embedded.  These 
rocks  are  called  Monchiquites. 

V.  THE  ULTRABASIC  ROCKS 

These  rocks  have  no  felspars,  and  in  most  of  them  the  quantity 
of  silica  is  below  45  %,  while  that  of  magnesia  is  from  35  to  48  %  ', 
they  are  composed  almost  entirely  of  ferro-magnesian  minerals. 

Limburgite  is  made  up  of  crystals  of  augite,  olivine,  and  mag- 
netite, embedded  in  a  glassy  ground  mass. 

Augitite  is  a  similar  rock,  but  without  olivine. 

Pyroxenite  is  a  holocrystalline,  plutonic  rock  composed  of  one 
or  more  varieties  of  pyroxene. 

Hornblendite  is  a  similar  rock  made  up  of  hornblende. 

The  Peridotites  are  likewise  plutonic  rocks  which  are  principally 
composed  of  olivine,  with  iron  ore  and  some  of  the  pyroxenes  or 
hornblende. 

The  Serpentine  Rocks  are  products  of  decomposition,  and  many 
of  them  have  been  formed  from  the  peridotites,  though  some 
are  derived  from  augitic  rocks,  such  as  gabbro,  and  others  from 
hornblendic  rocks.  In  rarer  instances  they  have  arisen  from  the 
alteration  of  acid  rocks. 


PYROCLASTIC   ROCKS  203 

APPENDIX 

THE   PYROCLASTIC   ROCKS 

THESE  rocks  are  formed  out  of  the  fragmental  materials  ejected 
from  volcanoes.  The  materials  are  of  course  igneous,  but  the 
rocks  themselves  differ  from  the  typical  igneous  rocks  in  several 
important  respects.  They  have  not  been  formed  in  their  present 
state  of  aggregation  by  cooling  from  a  molten  mass,  and  in  many 
cases  they  are  more  or  less  distinctly  stratified.  It  seems  best, 
therefore,  to  group  them  separately,  under  the  name  pyrodastic. 

Volcanic  Agglomerate  or  Breccia  is  a  mass  of  angular  blocks  of 
lava,  with  which  may  be  mingled  fragments  of  sedimentary  rocks, 
which  the  volcano  has  torn  off  from  the  sides  of  its  chimney.  The 
blocks  may  be  loose  or  cemented  together  into  hard  rock  by  a 
filling  of  finer  materials.  Ordinarily  the  breccia  is  formed  only 
near  the  vent,  but  sometimes  it  is  developed  on  a  great  scale,  as 
in  the  eastern  part  of  the  Yellowstone  Park. 

Tuffs  are  masses  of  volcanic  ashes  and  dust,  which  accumulate 
in  beds,  either  on  the  land  or  in  bodies  of  water.  Even  in  falling 
through  the  air,  the  particles  are  sorted,  in  some  degree,  in 
accordance  with  their  size,  and  the  tuffs  are  thus  usually  stratified, 
and  sometimes  have  fossils  in  them.  When  accumulated  under 
water,  the  ashes  are,  of  course,  stratified  and  may  be  mingled  with 
more  or  less  sedimentary  debris.  Such  subaqueous  tuffs  pass  into 
the  ordinary  sedimentary  rocks,  by  the  gradual  diminution  of  the 
volcanic  material.  When  examined  under  the  microscope,  even 
the  finest  tuffs  are  found  to  consist  of  crystals  and  particles  of  glass. 

The  volcanic  breccias  and  tuffs  may  best  be  classified  in  accord- 
ance with  the  nature  of  the  component  fragments.  Thus,  we  find 
rhyolite  tuffs  and  breccias,  andesite  tuffs  and  breccias,  basaltic 
tuffs  and  breccias,  and  the  like. 


CHAPTER   XI 
THE  SEDIMENTARY  ROCKS 

THE  materials  of  which  the  sedimentary  rocks  are  composed 
were,  in  the  first  instance  at  least,  derived  from  the  chemical 
decay  or  mechanical  abrasion  of  the  igneous  rocks,  and  hence 
they  are  often  called  derivative  or  secondary.  They  have  been 
laid  down  under  water  (or,  in  a  few  instances,  on  land)  and  are 
therefore  always  stratified  and,  for  the  most  part,  are  composed  of 
rounded  fragments,  seldom  crystalline. 

Almost  all  the  materials  which  we  have  found  in  the  igneous 
rocks  also  occur,  in  a  more  or  less  worn  and  comminuted  condi- 
tion, in  the  sedimentary  class.  However,  with  the  exception  of 
quartz,  the  great  bulk  of  the  sedimentary  materials  consists  of 
simpler  and  more  stable  compounds  than  the  igneous  minerals, 
from  the  decomposition  of  which  they  have  been  derived.  The 
principal  minerals  which  compose  the  sedimentary  rocks  are  quartz 
(SiO2),  clay  (A12O3,  2  SiO2,  2  HX)),  and  the  carbonate  and  sul- 
phate of  lime  (CaCO3,  CaSO4). 

Quartz  is  a  very  simple  and  stable  chemical  compound,  and 
hence,  in  the  ordinary  process  of  rock  decay,  it  remains  unchanged 
further  than  being  broken  up  into  smaller  pieces  and  rounded  by 
the  action  of  wind  or  running  water.  Clay  is  derived  principally 
from  the  decay  of  the  felspars,  and  the  lime  compounds  from  the 
complex  silicates  containing  lime,  which  are  so  frequent  in  the 
igneous  rocks.  These  rocks  also  yield  the  iron  oxides  which  are  so 
widely  diffused  in  the  sedimentary  class,  though  comparatively  sel- 
dom in  any  very  great  quantity.  Very  many  varieties  of  rocks 
are  produced  by  the  mixture  of  the  siliceous  (quartz),  argillaceous 
(clay),  and  calcareous  (lime)  materials  in  varying  proportions. 
The  sorting  out  of  material  by  water,  according  to  its  chemical 

204 


SILICEOUS   ROCKS  205 

nature,  is  usually  imperfect,  and  changes  from  point  to  point,  so 
that  the  sedimentary  rocks  have  an  even  less  definite  chemical 
composition  than  have  the  igneous. 

The  most  useful  classification  of  the  sedimentary  rocks  is,  pri- 
marily, according  to  the  mode  of  their  formation,  and  secondarily, 
according  to  their  composition.  This  gives  two  principal  divi- 
sions :  I,  the  Aqueous  Rocks,  or  those  laid  down  under  water ; 
II,  the  sEolian  Rocks,  those  which  were  accumulated  on  land, 
which  are  of  very  limited  extent  and  importance. 

The  aqueous  rocks  may  be  further  divided  into  three  classes  : 
i,  Mechanical  Deposits;  2,  Chemical  Precipitates;  3,  Organic 
Accumulations. 

I.   AQUEOUS  ROCKS 

The  rocks  laid  down  under  water  form  by  far  the  largest  and 
most  important  of  the  sedimentary  series. 

I.     MECHANICAL   DEPOSITS 

These  have  resulted  from  the  accumulation  of  debris  derived 
from  the  destruction  of  preexisting  rocks,  carried  in  mechanical 
suspension  by  moving  water,  whether  waves,  currents,  or  streams, 
and  dropped  when  the  velocity  of  the  moving  water  was  no  longer 
sufficient  to  carry  them.  The  study  of  the  dynamical  processes 
has  already  taught  us  that  such  accumulations  are  forming  to-day 
in  all  kinds  of  bodies  of  water,  and  an  examination  of  the  rocks 
will  show  that  similar  accumulations  have  been  made  since  the 
beginning  of  recorded  geological  time.  Mineralogically,  the 
mechanical  deposits  are  of  two  principal  kinds,  the  siliceous  and 
the  argillaceous.  The  sorting  power  of  water  has  been  sufficient 
to  separate  them  roughly,  though  we  find  mixtures  of  the  two  in 
all  proportions. 

a.   Siliceous  Rocks 

In  these  rocks  the  principal  component  is  quartz  in  fragments 
of  greater  or  less  size,  either  angular,  or  more  or  less  rounded  by 
wear.  Of  the  common  rock-forming  minerals  quartz  is  the  hardest 


206  THE   SEDIMENTARY    ROCKS 

and  the  one  which  best  resists  chemical  change.  Small  quantities 
of  other  minerals,  such  as  magnetite,  mica,  felspar,  garnet,  etc.,  are 
generally  present. 

Sand  is  made  up  of  fine  grains  of  quartz,  not  compacted  to- 
gether, but  forming  a  loose,  incoherent  mass.  River  sands  and 
those  formed  by  the  atmospheric  disintegration  of  rocks  commonly 
have  angular  grains,  due  to  the  splitting  up  of  the  quartz  fragments 
along  preexisting  flaws.  Beach  sands  are  more  apt  to  be  rounded, 
due  to  the  constant  wash  of  the  surf. 

Sandstone  is  a  rock  of  varying  degrees  of  hardness,  the  grains 
of  sand  being  held  together  by  a  cement.  The  most  important 
cementing  substances  are  carbonate  of  lime,  the  oxides  of  iron, 
and  silica.  The  sandstones  with  calcareous  cement  usually  yield 
quickly  to  the  action  of  the  weather,  because  of  the  solubility  of 
the  cement.  Those  with  ferruginous  cement  are  much  more 
durable  and  more  highly  coloured,  being  of  various  shades  of  red, 
yellow,  and  brown.  Most  durable  of  all  are  the  siliceous  cements. 

NovactiMte  (or  oilstone)  is  an  exceedingly  dense  and  fine-grained 
sandstone,  the  particles  of  which  are  as  fine  as  those  of  clay.  Its 
smoothness  and  hardness  fit  it  admirably  for  sharpening  fine  tools. 
Extensive  deposits  of  this  rock  occur  in  Arkansas. 

Varieties  of  sandstone  are  produced  by  the  conspicuous  admixt- 
ure of  other  minerals ;  thus,  micaceous  sandstone  has  abundant 
flakes  and  spangles  of  mica  deposited  along  the  planes  of  strati- 
fication. Argillaceous  sandstone  is  composed  of  a  more  finely 
grained  sand  than  the  more  typical  sandstones,  contains  consider- 
able quantities  of  clay,  and  is,  in  general,  more  thinly  bedded. 
The  flagstones,  so  largely  used  for  pavement,  are  examples  of  such 
a  rock,  and  split  readily  into  slabs  of  almost  any  desired  size. 

Arkose  or  Felspathic  Sandstone  is  a  rock  composed  largely  of 
cemented  grains  and  fragments  of  felspar,  which  have  been  me- 
chanically broken  up  by  the  action  of  water,  but  not  chemically 
disintegrated.  More  or  less  sand  is  often  mingled  with  the  felspar 
grains. 

Breccia  is  a  rock  made  of  large  angular  fragments  cemented 
together.  The  fragments  may  be  of  any  kind  of  material. 


ARGILLACEOUS    ROCKS  2O/ 

Gravel  is  composed  of  rounded,  water-worn  pebbles,  varying 
in  size  from  a  pin-head  up  to  cobblestones  and  boulders.  The 
coarser  kinds  are  often  called  shingle.  Gravel  may  be  composed 
of  almost  any  kind  of  rock  material,  but  the  commonest  pebbles 
are  of  quartz,  because  of  its  greater  resistance  to  wear.  Masses 
of  quartz  will  be  only  rounded  into  pebbles,  when  other  substances 
are  ground  into  fine  silt,  or  chemically  disintegrated,  and  so  washed 
into  deeper  water. 

Conglomerate  is  a  firm  rock,  made  up  of  pebbles,  embedded  in 
a  matrix  of  finer  material,  very  generally  sand.  As  above  re- 
marked with  regard  to  gravel,  the  component  pebbles  of  a  con- 
glomerate may  be  derived  from  any  kind  of  hard  rock,  but  siliceous 
pebbles  are  of  most  frequent  occurrence.  Different  names  are 
given  to  conglomerate,  according  to  the  character  of  the  pebbles, 
as  quartz  conglomerate,  flint  conglomerate,  limestone  conglomerate, 
granite  conglomerate,  etc. 

b.   Argillaceous  Rocks 

These  rocks  contain  a  greater  or  less  proportion  of  clay,  but 
nearly  always  with  large  admixtures  of  other  substances,  such  as 
exceedingly  fine  sand,  felspathic  mud,  and  the  like.  The  particles 
of  these  rocks  are  extremely  fine  and  are  carried  for  long  distances 
before  settling  to  the  bottom.  Hence  the  muds  and  clays  are  dis- 
tributed over  wider  areas  than  the  gravels  and  sands,  and  deposits 
of  them  indicate  quieter  and,  usually,  but  not  always,  deeper  waters 
than  the  conglomerates  and  sandstones. 

Kaolin,  also  called  China  or  porcelain  clay,  is  a  nearly  pure 
white  clay,  which  is  formed  principally  from  the  decay  of  the  alka- 
line felspars  of  granitic  rocks. 

Potter's  Clay  is  a  somewhat  less  pure  variety,  having  a  consider- 
able quantity  (18  to  37%)  of  finely  divided  quartz,  and  small 
quantities  of  lime  and  iron. 

Brick  Clay  is  a  still  more  impure  mixture  of  sand  and  clay,  with 
lime,  magnesia,  iron,  potash,  and  soda.  Clays  with  considerable 
percentages  of  iron  bum  red  in  the  kiln,  from  the  oxidation  of 
their  iron  compounds  into  Fe2O3.  Ordinary  red  bricks  do  not 


20&  THE  SEDIMENTARY   ROCKS 

withstand  high  temperatures  and  cannot  be  used  for  the  lining  ot 
furnaces,  because  the  iron,  alkalies,  and  alkaline  earths  which  they 
contain  cause  the  bricks  to  disintegrate. 

Fire-clay  is  a  nearly  pure  mixture  of  sand  and  clay,  with  only 
traces  of  iron,  magnesia,  or  lime,  and  therefore  burns  to  white 
or  buff-coloured  bricks,  which  will  resist  very  high  temperatures. 
Fire-clays  occur  frequently  beneath  coal  seams,  representing  the 
ancient  soil  in  which  the  coal  plants  grew.  Such  ancient  fire-clays 
are  often  hard  rocks,  and  must  be  ground  up  before  using. 

Mudstone  is  a  rock  which  is  composed  of  solidified  clay  or  fel- 
spathic  mud,  or  a  mixture  of  the  two,  and  which  crumbles  rapidly 
into  mud  when  exposed  to  the  action  of  the  weather. 

Shale  is  a  finely  stratified  or  laminated  clay  rock,  formed  from 
the  solidification  of  mud  and  silt.  In  some  of  the  paper  shales 
there  are  as  many  as  thirty  or  forty  laminae  to  the  inch,  each  rep- 
resenting a  separate  process  of  deposition.  Shales  ordinarily  con- 
tain more  or  less  sand,  and  as  this  increases  in  quantity,  they  shade 
gradually  into  arenaceous  shales  and  argillaceous  sandstones,  or 
by  the  increase  of  calcareous  matter  into  limestones.  Bituminous 
shale  is  coloured  very  dark  or  black  by  the  carbonaceous  matter 
with  which  it  is  saturated.  When  distilled,  the  bituminous  shales 
yield  hydrocarbons,  and  are  of  considerable  economic  importance  ; 
the  carbonaceous  matter  may  be  of  either  animal  or  vegetable 
origin.  Shales  of  this  class  grade  into  coals. 

Marl  is  clay  containing  carbonate  of  lime,  which  rapidly  crum- 
bles on  exposure  to  the  weather. 

2.     CHEMICAL  PRECIPITATES 

Rocks  which  have  been  principally  or  entirely  formed  by  chemi- 
cal processes  are,  for  the  most  part,  of  locally  restricted  extent,  and 
are  not  at  all  comparable  to  the  great  masses  of  mechanical  and 
organic  sediments.  This  arises  from  the  fact  that  the  chemical 
processes  occur  in  a  conspicuous  way  only  around  the  mouths  of 
certain  classes  of  springs  (p.  128),  and  in  closed  bodies  of  water 
without  outlet  and  subject  to  evaporation  (p.  149). 

The  chemical  precipitates  may  be  classed  under  the  following 


CHEMICAL   PRECIPITATES  2OQ 

heads  :  a,  Precipitates  of  the  alkalies  and  alkaline  earths ;  b,  sili- 
ceous precipitates ;  c,  ferruginous  precipitates. 

a.   Precipitates  of  the  Alkalies  and  Alkaline  Earths 

Calcareous  Tufa  or  Sinter,  Travertine,  Stalactite,  Onyx  Marbles, 
are  all  forms  of  carbonate  of  lime  deposited  from  solution,  either 
around  the  vents  of  springs,  or  by  percolating  waters  in  limestone 
caverns  or  in  lakes.  These  deposits  are  made  of  crystallized  cal- 
cite  (or  aragonite),  are  very  pure,  and  usually  white,  and  more  or 
less  translucent,  though  they  may  be  stained  by  other  substances 
dissolved  with  the  lime.  In  structure  they  are  banded  and  show 
rings  of  growth,  which  distinguishes  them  from  the  organic  lime- 
stones. The  so-called  "  Mexican  onyx  "  or  "  onyx  marble  "  is  a 
beautifully  banded  travertine  derived  from  ancient  spring  deposits. 

Oolite  is  a  limestone  composed  of  minute  spherules  of  carbonate 
of  lime,  cemented  into  a  more  or  less  compact  mass,  somewhat 
resembling  fish-roe,  whence  is  derived  the  name,  meaning  "  egg 
rock."  The  spherules  are  made  up  of  concentric  layers  of  car- 
bonate of  lime,  deposited  from  solution  around  some  nucleus,  it 
may  be  a  particle  of  sand  or  dust,  or  a  calcareous  fragment.  The 
beach  rock  of  a  coral  reef  (p.  169)  is  made  in  this  fashion,  and 
calcareous  sinter  often  has  a  similar  structure.  When  the  spheres 
are  larger,  resembling  peas  in  size  and  shape,  the  rock  is  called 
pisolite. 

Gypsum  (CaSO4,  H2O)  is  a  rock  as  well  as  a  mineral  (see 
p.  23),  and  is  deposited  from  solution  in  salt  lakes  and  lagoons, 
in  which  evaporation  balances  the  influx  of  water  (p.  151).  When 
pure,  gypsum  is  white,  but  it  is  often  coloured  grey,  brown,  or  red, 
by  iron  stains,  and  it  may  even  be  black.  It  forms  compact, 
crystalline,  or  fibrous  beds,  looking  like  limestone,  but  much  softer 
and  not  effervescing  with  acid ;  portions  of  the  beds  may  consist 
of  transparent  selenite  crystals.  Gypsum  sometimes  occurs  in  the 
form  of  anhydrite  (CaSO4),  but  it  is  not  known  under  what  con- 
ditions the  anhydrous  sulphate  has  been  deposited  from  solution. 

Rock  Salt  (NaCl)  is  precipitated  by  evaporation  from  the  dense 
brine  of  salt  lakes  and  lagoons,  following  the  deposition  of  gypsum, 


210  THE   SEDIMENTARY   ROCKS 

which  explains  the  very  common  association  of  the  two  rocks 
in  successive  beds.  The  salt  may  be  present  only  as  an  ingredi- 
ent of  shale  (saline  shale),  or  may  form  thin  layers,  indicating 
brief  periods  of  deposition,  followed  by  freshening  of  the  water. 
Again,  it  may  occur  in  enormously  thick  masses,  the  result  of 
long-continued  precipitation.  One  such  mass,  near  Berlin,  ex- 
ceeds 5000  feet  in  thickness.  Rock  salt  is  often  very  pure,  and 
then  it  is  transparent  and  colourless ;  but  it  is  frequently  stained 
by  iron,  or  mingled  with  dust  blown  into  the  lake  or  lagoon  which 
deposited  the  salt,  or  mixed  with  clay  and  other  mechanical  sedi- 
ments. 

b.   Siliceous  Precipitates 

These  are  much  less  common  and  extensive  than  the  calcareous, 
and  are  formed  under  exceptional  conditions. 

Geyserite,  or  Siliceous  Sinter,  is  deposited  in  dense  and  hard 
masses  around  the  mouths  of  geysers,  partly  by  the  evaporation  of 
the  water  which  holds  the  silica  in  solution,  and  partly  by  the 
action  of  Algae  (see  p.  130).  Large  terraces  of  this  rock  have 
been  built  up  by  the  geysers  of  the  Yellowstone  Park.  Geyserite 
also  occurs  as  an  uncompacted  white  powder. 

Cherts  (Flint  or  Hornstone)  are  exceedingly  dense  and  fine- 
grained masses,  which  the  microscope  shows  to  be  made  up  of 
very  minute  grains  of  chalcedony  mixed  with  more  or  less  amor- 
phous silica  and  crystals  of  quartz.  The  mode  of  origin  of  these 
masses  is  not  at  all  well  understood,  but  is  believed  to  be  by 
precipitation  from  sea-water.  In  the  Lake  Superior  region  occur 
cherty  rocks  which  are  mixtures  of  chalcedony  and  carbonate  of 
iron,  from  which  the  iron  ores  are  derived  by  a  process  of  weath- 
ering. 

c.   Ferruginous  Precipitates 

These  rocks  may,  with  almost  equal  propriety,  be  classed  with 
those  of  organic  origin,  because,  as  we  have  already  learned  (p.  135), 
the  concentration  into  beds  of  the  iron,  which  is  so  widely  diffused 
through  nearly  all  rocks  and  soils,  is  chiefly  due  to  the  action  of 
decomposing  vegetable  matter.  But  as  the  action  is  by  means  of 


CALCAREOUS   ACCUMULATIONS  211 

the  chemical  effects  of  dead  plants,  and  not  by  the  activities  of 
living  ones,  it  will  be  best  to  retain  the  iron  precipitates  with  the 
chemically  formed  rocks.  Important  as  they  are  from  an  economic 
point  of  view,  iron-ore  beds  are  not  extensive  constituents  of  the 
earth's  crust,  and  should  be  regarded  as  minerals  rather  than  as 
rocks. 

Ironstone  is  a  general  name  for  the  various  ores.  Haematite 
occurs  in  strata  and  tilling  the  cavities  in  limestone  rocks.  Limon- 
ite  also  occurs  in  strata,  and  at  the  present  day  is  precipitated 
from  solution  in  lakes.  Siderite  is  found  in  beds,  or  mixed  with 
chert  or  in  clay  concretions.  Magnetite  is  associated  with  crys- 
talline rocks,  in  which  it  often  forms  great  masses  and  beds. 

3.    ORGANIC   ACCUMULATIONS 

The  organically  formed  rocks  are  those  whose  materials  were 
accumulated  by  living  beings,  on  the  death  of  which  more  or  less 
of  their  substance  was  preserved,  added  to  by  successive  genera- 
tions, and  finally  compacted  into  rock.  In  preceding  chapters 
we  have  read  of  these  processes  as  going  on  at  the  present 
time,  in  peat  bogs,  in  the  coral  reefs,  shell  banks,  limestone  pla- 
teaus, and  organic  oozes  of  the  ocean.  Similar  processes  have 
been  at  work  in  all  ages  of  the  earth's  history  since  the  first 
appearance  of  living  things,  and  very  extensive  rocks  have  thus 
been  built  into  the  solid  crust  of  the  globe.  An  exact  classifica- 
tion would  require  us  to  place  certain  of  those  rocks  among  the 
mechanical  sediments,  because  the  actual  work  of  accumulation 
was  performed  by  mechanical  agencies,  such  as  waves  and  cur- 
rents. But  it  will  be  more  convenient  to  examine  together  all 
those  rocks  which  are  principally  made  up  of  organic  materials, 
especially  as  it  is  not  always  easy  to  distinguish  the  results  of  one 
mode  of  formation  from  those  of  the  other. 

a.    Calcareous  Accumulations 

Limestone  is  a  very  abundant,  important,  and  widely  distributed 
rock,  the  commonest  of  the  organic  accumulations.  It  is  com- 
posed of  carbonate  of  lime  in  varying  degrees  of  purity,  hardness, 


212  THE   SEDIMENTARY   ROCKS 

fineness  of  grain,  and  crystalline  texture.  Sand  or  clay  is  fre- 
quently present  as  an  impurity,  and  by  an  increase  in  these  mate- 
rials, the  limestones  pass  gradually  into  sandstones  and  shales. 
In  some  varieties  of  limestone  the  organic  nature  of  the  rock  is 
most  obvious,  shells,  corals,  crinoid  stems,  and  the  like  being  con- 
spicuously shown,  especially  on  weathered  surfaces.  In  other 
kinds  the  microscope  is  required  to  make  this  organic  nature 
clear  ;  while  in  others,  again,  the  calcareous  materials  have  been  so 
ground  up  by  the  action  of  the  waves,  that  all  traces  of  organic 
structure  have  disappeared.  The  example  of  the  reef  rock  now 
forming  in  many  coral  reefs  (p.  168)  is  a  warning  that  the  absence 
of  even  microscopic  structure  in  a  limestone  cannot  be  relied  upon 
as  a  proof  that  the  rock  is  not  of  organic  origin. 

The  great  limestones  are  almost  entirely  of  marine  origin,  though 
quite  extensive  fresh-water  limestones  are  known.  The  chemically 
formed  ones  are  never  very  widely  extended,  though  they  may 
form  quite  thick  masses.  As  a  rule,  the  limestones  are  deposited 
in  deeper  water  than  the  sandstones  and  shales,  but  not  necessarily 
so,  freedom  from  large  amounts  of  terrigenous  sediments  being 
more  important  than  depth  of  water.  This  is  shown  by  the  great 
calcareous  banks  of  the  Gulf  of  Mexico  and  the  Caribbean  Sea 
(p.  172),  and  coral  reefs  are  always  formed  in  shallow  water  of 
less  than  twenty  fathoms  in  depth. 

The  classification  of  the  limestones  is  very  difficult,  and  cannot 
be  readily  made  on  any  single  principle ;  mode  of  formation, 
purity,  texture,  and  nature  of  organic  material,  all  being  employed 
for  the  purpose. 

Shell  Marl  is  an  incoherent  and  crumbling  rock,  formed,  prin- 
cipally, at  the  bottom  of  fresh-water  lakes  and  ponds,  by  the 
accumulation  of  shells ;  it  frequently  occurs  beneath  peat  bogs, 
and  is  an  indication  that  the  bog  arose  from  the  choking  up  of  a 
lake  by  vegetable  growth.  When  the  shells  are  cemented  into 
a  hard  rock  they  form  a  fresh-water  limestone. 

Chalk  is  a  soft  limestone  of  friable,  earthy  texture,  and  fre- 
quently very  pure  ;  in  colour  it  may  be  snowy  white,  pale  grey,  or 
buff.  The  microscope  reveals  the  fact  that  chalk  is  principally 


MAGNESIAN   LIMESTONE 


213 


FIG.  76.  — Chalk  from  Kansas.  X  45. 
(Drawn  from  a  photograph  by  the  Geo- 
logical Survey  of  Iowa.) 


composed  of  the  shells  of  Foraminifera,  and  closely  resembles  the 
foraminiferal  oozes  forming  to-day  at  the  bottom  of  the  sea 
(p.  1 76) .  A  chalky  deposit  may, 
however,  be  formed  from  the 
debris  of  corals  ground  up  by 
the  waves. 

Hydraulic  Limestone  contains 
a  considerable  quantity  of  clay, 
and  the  mortar  made  from  it 
has  the  property  of  setting  under 
water,  so  that  it  is  used  in  the 
manufacture  of  hydraulic  cement. 

The  ordinary  massive  marine 
limestones  are  named  from  the 
character  of  the  organic  ma- 
terial which  predominates  in  them.  Thus,  we  have  coral  lime- 
stone, foraminiferal  limestone,  made  up  of  the  shells  of  very  large 
extinct  forms  of  the  Foraminifera  (Fusulina,  Nummulites,  Orbito- 
lites,  etc.),  crinoidal  limestone,  shell  limestone,  and  the  like. 

Though  much  the  larger  part  of  the  limestones  is  of  animal  origin, 
yet  certain  seaweeds  contribute  extensively  to  formation  of  these 
rocks,  and  there  is  much  reason  to  believe  that  chemical  precipita- 
tion is  of  greater  or  less  importance  in  nearly  all  varieties  of  the  rock. 

Dolomite,  or  Magnesian  Limestone,  is  a  compact,  granular  rock 
of  white,  grey,  or  yellow  colour,  composed  of  the  carbonates  of 
lime  and  magnesia.  Nearly  all  limestones  contain  some  carbonate 
of  magnesia,  but  the  name  dolomite  is  given  only  to  those  with  a 
considerable  percentage  of  that  substance  (5  to  20%).  How  far 
this  rock  is  made  up  of  the  mineral  dolomite,  and  how  far  it  is 
merely  a  mixture  of  the  two  carbonates,  is  uncertain,  as  is  also  the 
way  in  which  the  rock  was  formed.  Dolomite  contains  a  much 
larger  proportion  of  magnesia  than  the  shells  or  tests  of  any  known 
animals,  and  this  ingredient  must  therefore  have  been  added  after 
the  accumulation  of  the  calcareous  organisms.  Opinions  differ  as 
to  just  how  this  has  been  accomplished,  but  probably  the  magnesia 
has  been  derived  from  the  strong  brine  of  lagoons  and  salt  lakes. 


214  THE   SEDIMENTARY   ROCKS 

The  frequent  association  of  dolomite  with  gypsum  gives  additional 
probability  to  this  view.  A  similar  process  has  been  observed  in 
the  lagoons  of  coral  reefs  at  the  present  time  (p.  170),  and  it  has 
been  shown  that  dolomitization  takes  place  much  more  readily 
when  the  CaCO3  is  in  the  form  of  aragonite,  as  is  the  case  in  the 
shells  and  tests  of  many  marine  animals. 

Green  Sand  is  not  strictly  a  calcareous  deposit,  but  has  a  natural 
connection  with  that  series  of  rocks.  Green  sand  is  seen  by  the 
microscope  to  be  largely  composed  of  internal  casts  of  foraminiferal 
shells  in  the  mineral  glauconite  (p.  22).  The  dead  foraminiferal 
shells  which  lie  upon  certain  areas  of  the  ocean  floor  are  gradually 
filled  up  with  glauconite,  and  then  the  shells  are  dissolved,  leaving 
the  grains  of  the  mineral,  which  retain  the  form  into  which  they 
were  moulded.  This  process  is  still  going  on,  and  has  been  ob- 
served at  several  points  (p.  175). 

b.   Siliceous  Accumulations 

The  siliceous  deposits  of  organic  origin  are  very  much  less  com- 
mon and  less  extensively  developed  than  the  calcareous,  because 
of  the  relatively  small  amount  of  silica  which  is  in  solution  in 
ordinary  waters,  and  of  the  comparatively  few  organisms  which 
secrete  shells  or  tests  of  it.  Nevertheless,  these  beds  are  of  suf- 
ficient importance  to  require  mention. 

Infusorial  Earth  is  a  fine  white  powder  composed  of  the  micro- 
scopic tests,  or  frustules  of  the  minute  plants  called  diatoms.  The 
fineness  ajid  excessive  hardness  of  the  particles  make  this  an 
excellent  polishing  powder.  Beds  of  this  earth  occur  in  both 
marine  and  fresh-water  deposits.  At  Richmond,  Virginia,  is  a 
celebrated  deposit  of  this  kind. 

Siliceous  Oozes  are  exceedingly  rare  as  rocks  of  the  land ;  they 
consist  of  the  tests  of  Radiolaria,  such  as  are  now  accumulating 
in  the  deeper  parts  of  the  ocean  (p.  179).  The  only  land  areas 
in  which  such  deposits  have  been  found  occur  in  certain  of  the 
West  Indian  Islands  (Barbadoes,  Cuba,  and  others). 

Flint  or  Chert  occurs  in  nodules  or  beds,  especially  in  marine 
limestones,  though  it  is  also  found  among  the  sands  and  clays  of 


COAL  2 1 5 

certain  fresh-water  formations,  as  in  Wyoming.  Microscopic 
examination  sometimes  reveals  the  presence  of  sponge  spicules 
and  other  siliceous  organisms,  but  this  is  by  no  means  always  the 
case.  As  we  have  seen,  the  structureless  cherts  are  believed  to 
have  been  formed  by  chemical  precipitation  (p.  212). 

c.   Ferruginous  Accumulations 

The  iron  deposits  which  can  be  referred  to  the  activity  of  living 
creatures  are  of  small  extent  and  importance,  but  certain  of  the 
bog-iron  ores  are  believed  to  be  due  to  the  agency  of  diatoms, 
which  extract  the  iron  from  its  dissolved  state. 

d.    Carbonaceous  Accumulations 

The  rocks  of  this  group  are  formed,  almost  entirely,  by  the 
accumulation  of  vegetable  matter  and  its  progressive,  though  incom- 
plete, decay  under  water.  This  decay  is  of  such  a  nature  that  the 
gaseous  constituents  diminish,  while  the  carbon  is  removed  much 
less  rapidly,  consequently  the  proportion  of  the  latter  substance 
steadily  rises.  All  the  varieties  of  carbonaceous  rocks  pass  into 
one  another  so  gradually,  that  the  distinction  between  them  seems 
somewhat  arbitrary.  From  fresh  and  unchanged  vegetable  matter 
to  the  hardest  anthracite  there  is  an  unbroken  series  of  transitions. 

Peat  is  a  partially  carbonized  mass  of  vegetable  matter,  brown 
or  black  in  colour  and  showing  its  vegetable  nature  on  the  most 
superficial  examination,  though  the  parts  which  have  been  longest 
macerated  are  often  as  homogeneous  and  as  fine  grained  as  clay, 
and  reveal  their  true  nature  only  under  the  microscope. 

Lignite  or  Brown  Coal  is  a  brown  or  black  mass  of  mineralized 
and  compressed  peat,  and  though  still  plainly  showing  its  vegetable 
nature,  it  does  so  less  obviously  than  peat,  being  more  carbonized. 
It  is  an  inferior  fuel,  though  often  very  valuable  in  regions  where 
other  fuel  is  scarce  or  entirely  wanting. 

Coal  is  a  compact  dark  brown  or  black  rock,  in  which  vegetable 
structure  cannot  be  detected  by  the  unassisted  eye,  though  micro- 
scopic inspection  seldom  fails  to  reveal  it.  Coal  is  found  in  beds  or 
strata,  interstratified  with  shales,  sandstones,  and,  less  commonly, 


2l6  THE   SEDIMENTARY   ROCKS 

limestones.  The  different  kinds  of  coal  vary  much  in  hardness  and 
chemical  composition,  but  they  are  all  connected  by  intermediate 
gradations.  Bituminous  Coal  has  (neglecting  the  ash)  70  to  75  % 
of  carbon  and  25  to  30%  of  volatile  matters,  chiefly  hydrocarbons, 
which  are  driven  off  on  destructive  distillation.  Under  the  term 
bituminous  are  included  many  varieties  of  coal,  which  differ 
much  in  their  behaviour  and  in  their  value  for  different  purposes. 
Anthracite  is  a  hard,  lustrous  coal,  that  is  nearly  pure  carbon 
(aside  from  the  ash)  and  has  little  or  no  volatile  matter ;  it  burns 
without  smoke  or  flame  and  gives  an  intense  heat.  Semibitumi- 
nous  or  Steam  Coal  is  intermediate  in  character  and  composition 
between  the  bituminous  and  anthracite  varieties. 

Cannel  Coal  does  not  belong  in  the  series  of  coals  above  enu- 
merated, but  forms  a  very  distinct  variety.  It  occurs  in  lenticular 
patches,  not  in  beds,  and  is  very  compact,  though  not  very  hard  or 
heavy.  This  coal  has  from  70  to  85  %  of  carbon  and  the  high  propor- 
tion of  6  to  7%  of  hydrogen,  giving  off  large  quantities  of  gas  when 
heated,  and  burning  with  a  white,  candle-like  flame.  Even  with 
the  microscope,  it  is  difficult  to  detect  the  vegetable  structure  of 
cannel,  so  thoroughly  has  the  material  been  macerated.  Evidently, 
cannel  is  an  exceptional  coal  and  has  been  formed  in  a  somewhat 
peculiar  way.  While  the  ordinary  coals  evidently  represent  ancient 
peat  bogs,  which  by  subsidence  allowed  the  sea,  or  other  body  of 
water,  to  overflow  them  and  were  thus  sealed  up  and  buried  under 
sedimentary  deposits,  cannel  was  formed  in  pools  of  clear  water, 
in  which  vegetable  matter  was  accumulated  and  very  completely 
disintegrated.  This  is  shown  not  only  by  the  shape  of  the  coal 
patches,  but  also  by  the  fossil  fish  not  infrequently  found  in  cannel. 

The  following  table  (from  Kemp)  displays  the  composition  of 
the  typical  varieties  of  coal,  not  including  the  ash  :  — 

c.  H.  o.  N. 

Wood 50  6  43  i 

Peat 59  6  33  2 

Lignite 69  5.5  25  0.8 

Bituminous  Coal 82  5  13  0.8 

Anthracite 95  2.5  2.5  trace 


SOIL  217 


II.     AEOLIAN    ROCKS 

The  rocks  formed  on  dry  land  form  very  little  of  the  earth's 
crust,  in  this  respect  being  altogether  insignificant ;  their  impor- 
tance lies  in  the  hints  which  they  often  give  as  to  the  physical 
geography  of  the  place  and  time  of  their  formation. 

Blown  Sand  is  heaped  up  by  the  wind  into  dunes,  and  displays 
an  irregular  kind  of  stratification.  The  sand-grains,  abraded  by 
their  contact  with  hard  substances,  are  smaller,  more  rounded, 
and  less  angular  than  the  grains  of  river  or  even  beach  sands.  If 
the  sand  contains  any  considerable  quantity  of  calcareous  matter, 
the  dissolving  and  redeposition  of  this  by  percolating  waters  will 
bind  the  loose  material  into  quite  firm  rock. 

Talus  Blocks  gather  at  the  foot  of  cliffs  in  large  masses ;  these 
may  be  cemented  into  a  breccia  by  calcareous  deposits,  or,  by 
subsidence,  may  be  buried  in  marine  deposits. 

Soil.  —  In  Chapter  IV  it  was  shown  that  soil  is  mainly  the 
residual  product  left  by  the  atmospheric  decay  of  rocks,  and  that 
its  surface  layers  contain  more  or  less  organic  matter  and  are  filled 
with  the  roots  of  plants.  Soils  may  be  buried  under  aqueous 
deposits  by  floods,  or  by  subsidence  marine  deposits  may  be  built 
up  upon  the  soils,  which  are  then  interstratified  with  marine  rocks. 
Ancient  soils  have  been  frequently  preserved  in  this  manner,  filled 
with  fossil  roots,  and  sometimes  with  the  stumps  of  trees  still  stand- 
ing upon  them. 

In  logical  order,  the  Metamorphic  Rocks  would  next  come  up 
for  consideration ;  but  since  we  have,  as  yet,  learned  nothing  of 
the  processes  by  which  these  rocks  are  formed,  it  will  be  best  to 
defer  the  study  of  this  class  to  a  future  chapter,  when  the  rocks 
and  their  mode  of  formation  will  be  examined  together. 


CHAPTER  XII 
THE  STRUCTURE  OF  ROCK  MASSES  — STRATIFIED  ROCKS 

IN  the  preceding  chapter  we  have  studied  the  rocks  which  make 
up  the  crust  of  the  earth,  so  far  as  that  is  accessible  to  observation. 
It  remains  for  us  to  inquire  how  these  rocks  are  arranged  on  a 
large  scale,  and  to  what  displacements  and  dislocations  they  have 
been  subjected  since  the  time  of  their  formation.  Examined  with 
reference  to  the  simplest  and  broadest  facts  of  structure,  we  find 
that  rock  masses  fall  into  two  categories  :  (i)  Stratified  Rocks,  and 
(2)  Unstratified  or  Massive  Rocks.  A  very  brief  examination  will 
show  us  that  these  two  categories  correspond  respectively  to  the 
sedimentary  and  igneous  divisions  of  the  classification  according  to 
mode  of  origin,  neglecting,  for  the  present,  the  metamorphic  class. 

We  shall  begin  our  study  of  rock  masses  with  the  stratified 
series,  because  their  structure  and  mode  of  occurrence  are,  on  the 
whole,  the  simplest  and  most  intelligible,  and  tell  their  own  story. 
The  unstratified  series,  on  the  other  hand,  can  be  understood  only 
by  determining  their  relation  to  the  former. 

The  stratified  rocks  form  more  than  nine-tenths  of  the  earth's 
surface,  and  if  the  entire  series  of  them  were  present  at  any  one 
place,  they  would  have  a  maximum  thickness  of  about  thirty  miles, 
but  no  such  place  is  known.  The  regions  of  greatest  sedimentary 
accumulation  are  the  shallower  parts  of  the  oceans,  while  those 
regions  which  have  remained  as  dry  land,  through  long  ages,  have 
not  only  had  no  important  additions  to  their  surfaces,  but  have 
lost  immense  thicknesses  of  rock  through  denudation.  The  great 
oceanic  abysses  are  also  areas  of  excessively  slow  sedimentation, 
and  thus  the  thickness  of  the  stratified  rocks  varies  much  from 
point  to  point,  a  variation  which  has  been  increased  by  the  irregu- 
larities of  upheaval  and  depression  and  of  different  rates  of  denu- 

218 


STRATI  FIC  ATION  2 1 9 

dation.  Even  with  this  irregularity  in  the  formation  and  removal 
of  the  stratified  rocks,  it  would  be  exceedingly  difficult,  if  not 
impossible,  to  investigate  the  entire  series  of  them,  if  they  had  all 
retained  the  original  horizontal  positions  in  which  they  were  first 
laid  down.  In  many  places,  however,  the  rocks  have  been  steeply 
tilted  and  then  truncated  by  erosion,  so  that  their  edges  form  the 
surface  of  the  ground,  and  thus  great  thicknesses  of  them  may  be 
examined  without  descending  below  the  surface. 

Stratification,  or  division  into  layers,  is  the  most  persistent  and 
conspicuous  characteristic  of  the  sedimentary  rocks.  In  studying 
the  sedimentary  deposits  of  the  present  day  (Chapter  VIII)  we 
learned  that  by  the  sorting  power  of  water,  the  heterogeneous 
material  brought  from  the  land  is  arranged  into  more  or  less 
homogeneous  beds,  separated  from  one  another  by  distinct  planes 
of  division,  and  the  same  thing  is  true  of  the  sedimentary  rocks 
of  all  ages.  This  division  into  more  or  less  parallel  layers  is 
called  stratification,  and  the  extent  to  which  the  division  is  car- 
ried varies  according  to  circumstances. 

A  single  member,  or  bed,  of  a  stratified  rock,  whether  thick  or 
thin,  is  called  a  layer,  though  for  purposes  of  distinction,  exces- 
sively thin  layers  are  called  lamina.  Each  layer  or  lamina  repre- 
sents an  uninterrupted  deposition  of  material,  while  the  divisions 
between  them,  or  bedding  planes,  are  due  to  longer  or  shorter 
pauses  in  the  process.  A  stratum  is  the  collection  of  layers  of 
the  same  mineral  substance,  which  occur  together  and  may  con- 
sist of  one  or  many  layers.  However,  the  term  is  not  always 
employed  in  just  this  sense  and  often  means  the  same  as  layer. 
The  passage  from  one  stratum  to  another  is  generally  abrupt  and 
indicates  a  change  in  the  circumstances  of  deposition,  either  in  the 
depth  of  water,  or  in  the  character  of  the  material  brought  to  a 
given  spot,  or  in  both.  So  long  as  conditions  remain  the  same, 
the  same  kind  of  material  will  accumulate  over  a  given  area,  and 
thus  immense  thicknesses  of  similar  material  may  be  formed.  To 
keep  up  such  equality  of  conditions,  the  depth  of  water  must 
remain  constant,  and  hence  the  bottom  must  subside  as  rapidly 
as  the  sediment  accumulates. 


220 


THE  STRUCTURE  OF   STRATIFIED   ROCKS 


Usually,  a  section  of  thick  rock  masses  shows  continual  change 
of  material  at  different  levels.  Figure  77  is  a  section  of  the  rocks 
in  Beaver  County,  Pennsylvania,  in  which  several  different  kinds  of 
beds  register  the  changes  in  the  physical  geography  of  that  area. 
At  the  bottom  of  the  section  is  a  coal  seam  (No.  i),  the  con- 
solidated and  carbonized  vegetable  matter  which  accumulated 
in  an  ancient  fresh-water  swamp.  Next  came  a  subsidence  of 
the  swamp,  allowing  water  to  flow  in,  in  which  were  laid  down 
shales  of  different  kinds  (No.  2).  This 
fine-grained  clay  rock,  in  this  particular  in- 
stance, probably  represents  quiet,  sheltered 
water,  rather  than  any  considerable  depth 
of  it.  The  accumulations  of  mud  eventually 
shoaled  the  water  and  enabled  a  second 
peat  swamp  to  establish  itself;  this  is  regis- 
tered in  the  second  coal  bed  (No.  3),  the 
thinness  of  which  indicates  that  the  second 
swamp  did  not  last  so  long  as  the  first. 
Renewed  subsidence  again  restored  marine 
conditions,  as  is  shown  by  the  layer  of  cri- 
noidal  limestone  (No.  4)  which  overlies  the 
second  coal  bed.  This  depression  produced 
a  greater  depth  of  water,  and  the  distance 
from  land  was  sufficient  to  prevent  the  influx 
FIG.  77. -Section  in  coal  of  terrigenous  sediment.  Next,  the  water 
measures  of  western  Penn-  was  shoaled  by  an  upheaval,  and  argilla- 
sylvania.  (White.)  ceous  sands  were  laid  down,  which  now  form 

the  flaggy  sandstones  (No.  5)  overlying  the  limestone.  The  twenty- 
five  feet  of  sandstone,  aided  by  the  continued  slow  rise  of  the  sea- 
bottom,  silted  up  the  water  and  allowed  a  third  peat  bog  to  grow, 
the  result  of  which  is  the  third  coal  seam  (No.  6),  while  a  repeti- 
tion of  the  subsidence  once  more  brought  in  the  water,  in  which 
were  laid  down  the  seventy  feet  of  gravel  at  the  top  of  the  sec- 
tion. In  this  fashion  the  succession  of  strata  records  the  changes 
of  land  and  sea  which  were  in  progress  while  those  strata  were 
forming. 


CHANGES   IN   STRATA  221 

Somewhat  similar  changes  in  the  strata  may  be  occasioned  by 
the  steady  lowering  of  a  land  surface  through  denudation.  This 
diminishes  the  velocity  of  the  streams,  which,  in  its  turn,  changes 
the  character  of  the  materials  which  the  rivers  bring  to  the  sea. 

We  have  no  trustworthy  means  of  judging  how  long  a  time  was 
required  for  the  formation  of  any  given  stratum  or  series  of  strata, 
but  it  is  clear  that  different  kinds  of  beds  accumulate  at  very 
different  rates.  The  coarser  materials,  like  conglomerates  and 
sandstones,  were  piled  up  much  more  rapidly  than  the  shales  and 
limestones  ;  so  that  equal  thicknesses  of  different  kinds  of  strata 
imply  great  differences  in  the  time  required  to  form  them.  Com- 
paring like  strata  with  like,  we  may  say  that  the  thickness  of  a 
group  of  rocks  is  a  rough  measure  of  the  time  involved  in  their 
formation,  and  that  very  thick  masses  imply  a  very  long  lapse  of 
time,  but  we  cannot  infer  the  number  of  years  or  centuries  or 
millennia  required. 

Geological  chronology  can  be  relative  only.  Such  a  relative 
chronology  is  given  in  the  section  that  we  have  examined  by  the 
order  of  succession  of  the  beds.  Obviously  the  lowest  stratum  is 
the  oldest  and  the  one  at  the  top  the  newest.  This  may  be  put  as 
a  general  principle,  that,  unless  strata  have  lost  their  original  posi- 
tion through  disturbance  or  dislocation,  their  order  of  superposi- 
tion is  their  order  of  relative  age.  It  is  for  this  reason  that  in 
geological  sections  the  strata  are  numbered  and  read  from  below 
upward. 

Change  in  the  character  of  the  strata  takes  place  not  only  verti- 
cally, but  also  horizontally,  since  no  stratum  is  universal,  even  for 
a  single  continent.  Our  study  of  the  processes  of  sedimentation 
which  are  now  at  work,  showed  us  that  the  character  of  the  bottom 
in  the  ocean  or  in  lakes  is  subject  to  frequent  changes,  varying 
with  the  depth  of  water  and  other  factors.  The  same  is  true  of 
the  ancient  sea  and  lake  bottoms,  now  represented  by  the  strati- 
fied rocks  of  the  land.  Strata  may  persist  with  great  evenness 
and  uniform  thickness  over  vast  areas,  and  in  such  cases  the  bed- 
ding planes  remain  sensibly  parallel.  But  sooner  or  later,  the  beds, 
whenever  they  can  be  traced  far  enough,  are  found  to  thin  out  to 


222 


THE   STRUCTURE   OF   STRATIFIED    ROCKS 


FlG.  78.  —  Sections  near  Colorado  Springs.     (Hayden.) 


CHANGES   OF   STRATA 


223 


edges  and  to  dovetail  in  with  beds  of  a  different  character.  When 
the  strata  are  of  constant  thickness  for  considerable  distances,  and 
the  bedding  planes  remain  parallel,  the  stratification  is  said  to  be 
regular.  In  many  cases  these  changes  take  place  rapidly  from 


FlG.  79.  —  Cross-bedded  sandstone.    (Photograph  by  the  Iowa  Geological  Survey.) 

point  to  point,  and  then  the  strata  are  plainly  of  lenticular  shape, 
thickest  in  the  middle,  thinning  quickly  to  the  edges.  Here  the 
bedding  planes  are  distinctly  not  parallel,  and  the  stratification  is 
irregular. 

An  example  of  rapid   horizontal  changes  is  given  in  the  two 
accompanying  parallel  sections  (Fig.  78),  taken  through  the  same 


224  THE   STRUCTURE   OF   STRATIFIED    ROCKS 

beds,  only  twenty  feet  apart.  In  these  sections  the  differences  of 
thickness  of  the  coal  seams  and  of  the  sands  and  clays  which 
separate  them  are  very  striking. 

The  finer  details  of  structure  of  the  stratified  rocks  likewise 
afford  valuable  testimony  as  to  the  circumstances  under  which  the 
rocks  were  laid  down. 

Cross-bedding   (also    called  false   or  current  bedding)  is  pro- 


FlG.  80.  —  Ripple  marks  on  a  modern  sea-beach.     (U.  S.  G.  S.) 

duced  by  a  strong  current  pushing  sediment  along  the  bottom  and 
thus  bringing  about  an  oblique  lamination,  or  by  the  plunge  of  a 
wave  piling  up  material  in  heaps.  Cross-bedded  layers  frequently 
alternate  with  horizontally  bedded  ones,  formed  in  slack  water  or 
at  ebb  tide.  Cross-bedding  is  most  common  in  sandstones,  but 
it  may  occur  in  other  rocks,  even  in  limestones,  though  the  latter 
are  comparatively  seldom  accumulated  in  such  shallow  water. 


WAVE   MARKS 


225 


Wind  Drift  is  characteristically  different  from  cross-bedding  v 
here  the  laminae  form  all  sorts  of  angles  with  one  another.  The 
lamination  is  parallel  to  the  surface  as  that  was  at  the  time  each 
lamina  was  formed ;  but  the  sand  ridges  and  dunes  are  continually 
changing  their  shape,  as  the  force  and  direction  of  the  wind  vary, 
and  thus  a  very  complex  arrangement  results. 

Ripple  Marks,  exactly  like   those   on  any  sandy  beach  of  the 


:  mark  and  rain  prints,  modern  sandy  beach.     (U.  S.  G.  S.) 

present,  are  found  in  rocks  of  almost  all  geological  dates.  Such 
marks  are  formed  by  the  wind  or  by  the  rippling  movement  of 
shoal  water.  Ripple  marks  are  most  frequent  and  best  shown  in 
sandstones,  but  other  rocks,  such  as  shales,  often  exhibit  them 
also. 

Wave  Marks  are  formed  by  waves  washing  up  on  the  beach 

after  they  have  broken,  and  are  preserved  by  the  deposit  of  thin 

layers  of  sand  on  the  edges  of  the  waves,  and  indicate  that  the 

rock  in  which  such  marks  occur  was  formed  on  the  very  beach. 

Q 


226  THE   STRUCTURE   OF   STRATIFIED    ROCKS 

Naturally,  they  are  almost  confined  to  sandstones,  for  conglomer- 
ates are  too  coarse  to  retain  such  markings.  Wave  markings, 
ripple  marks,  and  cross-bedding  are  all  indications  of  very  shallow. 
water  and  of  the  immediate  proximity  of  the  shore. 

Rill  Marks  are  made  by  little  rills  of  water  trickling  over  the 
sand  as  the  tide  goes  out. 


FIG.  82.  —  Rill  marks  on  modern  sandy  beach.     (U.  S.  G.  S.) 

Sun-cracks  are  produced  in  mud  flats,  where  the  mud  and  silt, 
exposed  by  the  retreating  tide  to  the  heat  of  the  sun,  dry  and 
crack  open. in  more  or  less  regular  patterns,  a  process  which  may 
be  often  observed  in  drying  rain  pools.  When  the  incoming  tide 
advances  so  gently  as  not  to  disturb  the  cracked  and  hardened 
surface,  but  deposits  a  new  layer  upon  it,  filling  up  the  cracks,  the 
latter  will  be  preserved,  and  on  the  overlying  layer  the  casts  of  the 
cracks  will  stand  out  in  relief,  when  the  two  layers  are  separated. 
Such  marks  are  common  in  the  fine  argillaceous  sandstones  of  the 
Connecticut  valley,  New  Jersey,  and  southeastern  Pennsylvania. 


FOOTPRINTS 


227 


FIG.  83.  —  Sun-cracks  in  sandstone.     (Photograph  by  Pynchon.) 

Rain  Prints  are  sometimes  preserved  in  fine  sand  or  mud  in 
the  same  fashion.  The  raindrops  make  little  pits  on  the  soft 
surface,  which  are  circular  when  the  drops  fall  vertically,  or  oval 
and  with  edge  raised  on  one  side,  when  the  rain  is  driven  obliquely 
before  the  wind.  A  gentle  deposition  of  fine  sediment  upon  the 
pitted  surface  will  preserve  the  marks. 

Tracks  of  Animals  are  not  infrequently  found  in  the  strata,  and 
have  been  preserved  in  much  the  same  way.  Marine  animals,  such  as 
worms  and  crustaceans,  leave  tracks'  in  fine  sand,  which  are  covered 
so  gradually  and  gently  by  fresh  layers  that  the  marks  are  not 
disturbed.  Tracks  of  land  animals  may  be  preserved  when  made 
on  the  tidal  flats  of  sheltered  estuaries,  where  the  surface  is  some- 
what hardened  by  the  exposure  to  sun  and  air.  On  the  open  sea- 
beach  such  tracks  would  be  soon  obliterated  by  the  waves. 

Marks  of  this  latter  class,  sun-cracks,  rain  prints,  and  tracks  of 
land  animals,  show  that  the  surface  on  which  they  occur  was  exposed 
to  the  air,  either  periodically  by  the  ebb  and  flow  of  the  tides,  or 
at  stages  of  low  water  in  rivers  and  lakes.  They  are  illustrations 
of  the  way  in  which  conditions,  long  passed  away,  have  been 
recorded  for  ages  in  the  solid  rocks. 

Concretions,  or  Nodules,  are  developed  after  the  formation 
of  strata.  They  are  balls  or  irregular  lumps  of  material  differing 


228  THE   STRUCTURE  OF   STRATIFIED    ROCKS 


FIG.  84.  —  Markings  by  marine  worms,  modern. 


FIG.  85. —  Tracks  of  land  animal  and  sun-cracks,  on  slab  of  sandstone. 


CONCRETIONS 


229 


from  that  of  the  stratum  in  which  they  occur.  They  are  not  peb- 
bles, which  are  older  than  the  stratum  which  contains  them  and 
which  were  embedded  just  as  we  find  them,  but  are  younger  than 
the  stratum  and  were  formed  subsequently.  This  is  shown  by  the 
fact  that  the  planes  of  stratification  may  often  be  traced  through 
the  concretion,  and  that  fossils  are  sometimes  found  partly  within 
and  partly  without  the  nodule.  In  shape  the  concretions  vary 


FlG.   86.  —  Large   concretions,  weathered   out   of  sandstone,   near   Fort   Buford, 
Mont.     (U.  S.  G.  S.) 

greatly,  from  almost  true  spheres,  to  grotesque  aggregations,  but 
always  with  rounded  form,  and  almost  as  great  a  variety  of  mate- 
rial is  found  among  them.  Very  often  a  foreign  body,  like  a  fossil 
shell  or  leaf,  forms  the  centre  or  nucleus  of  the  nodule,  which  has 
been  built  up,  often  in  concentric  layers,  around  the  nucleus. 
One  form  of  concretion,  known  as  a  septarium,  is  divided  inter- 
nally by  radial  cracks,  which  were  subsequently  filled  up  with  some 
mineral  deposited  from  solution  by  percolating  waters. 

The  agency  which  produces  concretions  cannot  as  yet  be  ex- 


230 


DISPLACEMENTS   OF   STRATA 


plained.  The  material  of  which  they  are  made  must  have  been 
scattered  through  the  stratum  and  then  gathered  together  at  a 
later  period.  Such  nodules  have  been  observed  in  the  process  of 
formation  in  modern  sediments,  and  it  has  further  been  noticed 
that  when  finely  powdered  substances  are  mixed  together,  certain 
of  them  do  segregate  into  lumps.  These  observations,  however, 
merely  confirm  the  conclusion  that  concretions  are  due  to  segre- 
gation of  scattered  material  in  the  stratum,  they  give  us  no 
explanation  of  the  fact. 


-.    - 


FlG.  87. —  Ironstone  concretion,  split  open  to  show  the  fossil  leaf  which  forms  the 
nucleus.     Mazon  Creek,  Illinois. 

The  commonest  concretions  are  those  of  clay  in  various  kinds 
of  rock,  of  flint  and  chert  in  limestone,  and  of  ironstone  in  clay 
rocks. 

DISPLACEMENTS  OF  STRATIFIED  ROCKS 

It  is  evident  that  the  stratified  rocks  which  form  the  land  must 
have  been  changed,  at  least  relatively,  from  the  position  which 
they  originally  occupied,  since  the  great  bulk  of  them  were  laid 
down  under  the  sea.  Originally  they  must  have  been  nearly 
horizontal,  for  this  is  a  necessary  result  of  the  operation  of  gravity. 
Just  as  a  deep  fall  of  snow,  when  not  drifted  by  the  wind,  gradually 


KINDS   OF   DISPLACEMENT  231 

covers  up  the  minor  inequalities  of  the  ground  and  leaves  a  level 
surface,  so  on  the  sea-bottom,  the  sediments  are  spread  out  in 
nearly  level  layers,  disregarding  ordinary  inequalities.  We  must 
remember,  however,  that  this  original  horizontality  is  not  exact, 
and  departures  from  it  are  not  infrequent.  On  a  large  scale,  these 
departures  from  the  horizontal  position  are  very  slight,  while  those 
that  are  conspicuous  are  always  local. 

Examples  of  such  original  deviations  from  horizontality  are  the 
following:  (i)  When  a  sediment-laden  stream  or  current  empties 
abruptly  into  a  deep  basin  with  steeply  sloping  sides,  the  sediment 
is  rapidly  deposited  in  oblique  layers,  which  follow  the  slope  of 
the  sides.  (2)  Alluvial  cones,  or  fans  (p.  137),  have  steeply 
inclined  layers,  for  a  similar  reason.  Both  of  these  cases  resemble 
the  artificial  embankments  which  are  built  out  by  dumping  earth 
or  gravel  over  the  end,  until  each  successive  section  is  raised  to 
the  necessary  level.  In  such  embankments  the  obliquity  of  the 
layers  is  often  plainly  visible.  (3)  Sand  beaches  often  have  a 
considerable  inclination,  as  much  as  8  %,  and  newly  added  layers 
follow  this  slope.  (4)  On  a  large  scale,  the  great  sheets  of  sedi- 
ment that  cover  the  sea-bottom  generally  have  a  slight  inclination 
away  from  the  land,  with  a  somewhat  increased  slope  along  lines 
of  depression.  These  slight  original  inclinations  of  sedimentary 
masses,  either  as  a  whole,  or  along  certain  lines,  are  called  initial 
dips,  and  have  an  important  bearing  upon  the  results  of  subsequent 
movements  of  displacement. 

The  displacements  to  which  strata  have  been  subjected  after 
their  formation  are  of  two  principal  kinds:  (i)  In  the  first  kind, 
the  strata  have  been  lifted  vertically  upward,  often  to  great  heights, 
without  losing  their  horizontality.  Over  great  areas  of  our  Western 
States  and  those  of  the  Mississippi  valley,  the  beds  are  almost  as 
truly  horizontal  as  when  they  were  first  laid  down.  In  some  of 
the  lofty  plateaus  through  which  the  Grand  Canon  of  the  Colorado 
has  been  cut,  almost  horizontal  strata  are  found  10,000  feet  above, 
the  sea-level.  (2)  More  frequent  and  typical  are  the  displacements 
of  the  second  class,  by  which  the  beds  are  tilted  and  inclined  at 
various  angles,  sometimes  bringing  the  strata  into  a  vertical  posi- 


232 


DISPLACEMENTS   OF   STRATA 


FIG.  88.  -  Clinometer. 


tion,  and  occasionally  even  overturning  and  inverting  them.     In 
the  comparatively  small  exposures  of  strata  which  may  be  seen 

in  ordinary  sections  in  cliffs 
and  ravines,  the  rocks  appear 
to  be  simply  inclined,  and  the 
strata  themselves  to  be  nearly 
straight.  But  when  the  struct- 
ure is  determined  on  a  large 
scale,  it  is  found  that  this  ap- 
pearance is  due  to  the  limited 
area  visible  in  one  view,  and 
that  the  apparently  straight 
beds  are  really  portions  of  great 

CUrVCS-  Such  CUrV6S  are  Called 
folds. 

Dip.  —  The  angle  of  inclination  which  a  tilted  stratum 
makes  with  the  plane  of  the  horizon  is  called  the  dip,  and 
is  measured  in  degrees.  The  line  or  direction  of  the  dip  is 
the  line  of  steepest  inclination  of  the  dipping  bed,  and  is 
expressed  in  terms  of  compass  bearing.  For  example,  a 
stratum  is  said  to  have  a  dip  of  15°  to  the  northwest.  The 
angle  of  dip  is  measured  by  means  of  an  instrument  called 
a  clinometer,  of  which  many  kinds  are  in  use.  An  excellent 
form  of  this  instrument  is  shown  in  the  annexed  figure. 
AB  is  a  long,  rigid  arm  which  is  applied  to  the  upper  sur- 
face of  the  stratum,  the  dip  of  which  is  to  be  measured. 
AC,  the  shorter  arm,  carries  a  spirit  level,  and  is  pivoted  at 
A,  allowing  it  to  be  raised  or  lowered.  DE  is  a  graduated 
quadrant,  on  which  may  be  read  the  angle  included  between 
the  two  arms.  The  long  arm  is  laid  on  the  surface  of  the 
inclined  stratum,  and  the  short  arm  raised,  until  the  spirit 
level  shows  that  it  is  horizontal,  and  the  angle  is  then  read 
off  from  the  graduated  quadrant.  Figure  89  shows  that 
this  is  the  angle  of  dip.  AB  is  the  surface  of  the  dipping 
bed,  the  line  EBF  is  in  the  plane  of  the  horizon,  and  the 
line  CAD  is  the  horizontal  line  of  the  spirit  level,  parallel 


FOLDS  233 

to  EBF.  The  angle  DAB  is  the  one  actually  measured,  but  as 
the  two  horizontal  lines  are  parallel,  that  angle  must  be  equal  to 
the  angle  ABE,  which 

is  the  actual  angle  of     C 

dip. 

Strike,  — The  line  of 
intersection  formed  by 

the  dipping  bed  with      g 

the  plane  of  the  hori- 

t  IG.  89.  —  Diagram  explanatory  of  dip  measurement. 

zon  is  called  the  line 

of  strike  and  is  necessarily  at  right  angles  to  the  line  of  dip. 
(See  Fig.  91.)  If  a  piece  of  slate  be  held  in  an  inclined  position 
and  lowered  into  a  vessel  of  water,  the  wet  line  will  represent  the 
strike.  As  long  as  the  direction  of  the  dip  remains  constant,  the 
line  of  strike  is  straight,  but  as  the  direction  of  the  dip  changes, 
the  strike  changes  also,  always  keeping  at  right  angles  to  the  dip, 
and  in  such  cases  as  the  Appalachian  Mountains  the  lines  of  strike 
are  sweeping  curves. 

Outcrop  is  the  line  along  which  a  dipping  bed  cuts  the  surface  of 
the  ground,  and  is,  of  course,  due  to  erosion,  which  has  truncated 
the  folds  of  strata.  Except  in  the  case  of  fractured  beds,  which 
will  be  considered  in  the  following  section,  if  there  were  no  erosion, 
there  could  be  no  outcrop.  When  the  surface  of  the  ground  is 
level,  outcrop  and  strike  become  coincident,  because  the  surface 
then  is  practically  a  horizontal  plane.  With  the  dip  remaining  con- 
stant, the  more  rugged  and  broken  the  surface  becomes,  the  more 
widely  do  strike  and  outcrop  diverge.  For  a  given  form  of  surface, 
outcrop  and  strike  differ  more  when  the  beds  dip  at  a  low  angle 
than  when  the  dip  is  steep,  for  when  the  strata  are  vertical,  outcrop 
and  strike  again  coincide,  and  the  more  nearly  the  strata  approach 
verticality,  the  more  closely  do  the  two  lines  come  together. 

Having  digressed  to  make  these  necessary  definitions,  we  may 
now  return  to  the  subject  of  folds. 

Folds  present  themselves  to  observation  under  many  different 
aspects,  all  of  which  may  be  regarded  as  modifications  of  two 
principal  types. 


234 


DISPLACEMENTS   OF   STRATA 


(i)  The  Anticline  is  an  upward  fold  or  arch  of  strata,  from 
the  summit  of  which  the  beds  clip  downward  on  both  sides.  The 
curve  of  the  arch  may  be  broad  and  gentle,  or  sharp  and  angular, 
or  anything  between  the  two.  The  line  along  which  the  fold  is 
prolonged  is  called  the  anticlinal  axis  and  may  be  scores  of  miles 
in  length,  or  only  a  few  feet.  This  may  be  illustrated  by  an 
ordinary  roof,  which  represents  the  two  sides  or  limbs  of  the 


FlG.  90.  —  Anticline  on  the  Potomac,  Maryland.     (U.  S.  G.  S.) 

anticline,  while  the  ridge-pole  will  represent  the  anticlinal  axis. 
Whether  long  or -short,  the  fold  eventually  dies  away,  and  thus  the 
summit  of  the  arch  is  not  perfectly  level,  but  more  or  less  steeply 
inclined,  and  this  inclination  is  called  the  pitch  of  the  fold.  In 


DOMES   AND   BASINS 


235 


accordance  with  the  length  of  the  axis  and  the  steepness  of  the 
pitch,  the  uneroded  anticlinal  is  either  short  and  dome-like,  or 
elongate  and  cigar-shaped. 


FlG.  91.  —  Anticlinal  limb  of  fold.     P,  axis  pitching  to  the  left;  S  S,  line  of  strike; 
D,  line  of  dip.    The  dotted  line  is  the  plane  of  the  axis.     (Willis.) 

(2)  The  Syncline  is  the  complement  of  the  anticline,  and  in 
this  the  beds  are  bent  into  a  downward  fold  or  trough,  dipping 
from  both  sides  toward  the  bottom  of  the  trough,  which  forms  the 
longitudinal  synclinal  axis.  As  in  the  anticline,  the  axis  may  be 
long  or  short,  with  gentle  or  steep  pitch,  forming  long,  narrow, 
"canoe-shaped"  valleys,  or  oval,  even  round,  basins.  In  section 


FlG.  92.  —  Synclinal  limb  of  fold.     (Willis.) 

the  syncline  may  be  shallow  and  widely  open,  or  with  steep  sides 
and  angular  bottom. 

Domes  and  Basins  are  special  cases  of  anticlines  and  synclines. 
The  dome  is  an  anticlinal  fold,  in  which  the  axis  is  reduced 
to  zero,  the  dip  of  the  beds  being  downward  in  all  directions  from 
the  summit  of  the  dome.  As  the  dip  changes,  the  strike  changes, 


236 


DISPLACEMENTS   OF   STRATA 


describing  an  oval  or  circle.  Similarly,  the  basin  is  a  syncline 
with  axis  reduced  to  zero,  the  beds  dipping  downward  from  all 
sides  to  the  bottom  of  the  basin,  and  the  strike  forming  the  edge 
of  the  basin.  The  term  basin  is  used  in  different  senses,  and  it 
is  necessary  to  distinguish  carefully  between  a  basin  of  folding  and 
one  which  has  been  excavated  by  erosion. 

It  is  rare  to  find  a  single  anticline  or  syncline  occurring  by  itself; 
very  much  more  frequently  they  are  found  in  more  or  less  parallel 
series,  each  pair  of  anticlines  connected  by  a  syncline.  At  one 
end  of  the  system  we  may  find  several  axes  converging  and  unit- 
ing into  a  single  fold,  and  they  all  die  away  sooner  or  later,  the 
pitch  of  the  folds  coinciding  with  the  dip  of  the  beds. 

Anticlinorium  and  Synclinorium.  —  The  system  of  roughly  paral- 
lel folds  which  are  grouped  together  may  be,  when  regarded  as  a 


.  93.  —  Anticlinorium:  section  through  the  Appalachian  Mountains.     (Willis.) 


whole,  either  anticlinal,  rising  up  into  a  great  compound  arch,  or 
synclinal,  depressed  into  a  great  compound  trough.  The  former 
is  called  an  anticlinorium,  and  the  latter  a  synclinorium.  The 
secondary  folds  which  compose  one  of  these  systems  may  them- 


FlG.  94.  —  Synclinorium,  Mt.  Greylock,  Massachusetts.     (Dale.) 

selves  be  compound  and  made  up  of  many  subordinate  folds,  the 
smallest  of  which  can  be  detected  only  with  the  microscope. 

Geanticline  and  Geosyncline.  —  The  folds  and  flexures  which 
we  have  so  far  examined  are  those  which  affect  the  strata  at  the 
surface  or  at  comparatively  moderate  depths.  It  is  quite  impos- 
sible that  the  whole  crust  can  be  involved  in  folds  of  such  small 
amplitude.  The  crust  is,  however,  subject  to  flexures  of  its  own, 


GEOSYNCLINES 


237 


which  are  characterized  by  their  great  width  and  gentle  slope. 
Such  flexures  have  been  named  by  Dana  geanticlines  and  geo- 
synclines,  to  express  their  importance  for  the  earth  as  a  whole. 


FIG.  95.  —  Diagrams  of  folds.  (Willis.)  i.  Upright  or  symmetrical  open 
folds.  2.  Asymmetrical  fold,  open.  3.  Asymmetrical  fold,  closed  and  overturned. 
4.  Symmetrical  fold,  closed.  5.  Closed  anticline,  overturned.  6.  Closed  anticline, 
recumbent. 

The  great  thickness  of  sediments  which  form  the  Appalachian 
Mountains  (exceeding  30,000  feet)  was  laid  down  in  an  immense 
geosynclinal  trough,  which  through  long  ages  slowly  sank  as  the 
sediments  accumulated.  The  rate  of  subsidence  so  nearly  equalled 


238 


DISPLACEMENTS   OF   STRATA 


the  rate  of  deposition,  that  almost  the  entire  thickness  was  accumu- 
lated in  shallow  water,  as  is  indicated  by  the  character  of  the  rocks 
themselves.  Geanticlines  are  less  easy  to  detect,  but  there  is  evi- 
dence to  show  that  they  do  occur  on  an  equally  great  scale. 


FIG.  96.  —  Asymmetrical  open  fold.     High  Falls,  Ulster  County,  N.Y. 
(U.  S.  G.  S.) 

Folds  may  be  classified  either  in  accordance  with  the  relation 
which  their  opposite  limbs  bear  to  each  other,  or  with  reference 
to  the  degree  of  compression  to  which  they  have  been  subjected. 
Using  the  first  method,  we  may  distinguish  the  following  varieties. 

Upright  or  Symmetrical.  —  In  this  case  the  two  limbs  of  the  fold 
dip  at  the  same  angle,  the  plane  of  the  axis  of  the  flexure  is  verti- 


CLASSIFICATION   OF   FOLDS 


239 


cal  and  bisects  the  fold  into  equal  halves.  In  asymmetrical,  or 
inclined,  folds  the  opposite  limbs  have  different  angles  of  dip,  the 
axial  plane  is  oblique  and  divides  the  flexure  into  more  or  less  dis- 
similar parts.  When  one  limb  has  been  pushed  over  past  the  per- 
pendicular, the  fold  is  said  to  be  overturned  or  inverted,  and  when 


FlG.  97. —  Symmetrical,  closed  anticline:  near  Quebec,  Canada.     (U.  S.  G.  S.) 

this  has  gone  so  far  that  one  of  the  limbs  becomes  nearly  or  quite 
horizontal,  the  fold  is  recumbent. 

According  to  the  second  mode  of  classification,  we  have  a  some- 
what different  series  of  terms ;  but  both  methods  have  their  uses 
and  must  be  employed.  Open  folds  are  those  in  which  the  limbs 
are  widely  separated ;  strata  with  open,  gentle  flexures  are  said  to 
be  undulating.  Closed  folds  are  those  in  which  the  limbs  of  the 


240 


DISPLACEMENTS   OF   STRATA 


flexures  are  in  contact  and  any  further  compression  must  be  re- 
lieved by  a  thinning  of  the  beds.  Contorted  strata  are  thrown 
into  closed  folds,  which  are  connected  by  sharp,  angular  turns. 
Plications  are  intense  crumplings  and  corrugations  of  the  strata. 


FIG.  98.  — Closed  recumbent  folds,  Doe  River,  Tennessee.     (U.  S.  G.  S.) 

Isoclinal  folds  are  those  which  have  been  so  bent  back  on  them- 
selves that  the  limbs  of  the  flexures  are  all  parallel,  or  nearly  so. 
When  a  series  of  isoclines  has  been  planed  down  by  erosion  to  a 
level,  the  strata  show  a  continuous,  uniform  dip  and  present  a 
deceptive  appearance  of  being  a  simple,  tilted  succession  of  beds. 
A  still  further  compression  of  isoclinal  folds  produces  fan  folds. 
In  this  structure  the  anticlinal  is  broader  at  the  summit  than  at 


COMPLEX   FOLDS 


241 


the  base  and  the  synclinal  broader  below  than  above,  a  reversal 
of  the  normal  arrangement. 

The  isoclinal  and  fan  folds  may  be  upright,  inclined,  inverted, 
or  recumbent.  In  the  closed  folds  there  has  been  such  enor- 
mous compression  that  the  same  strata  are  of  different  thickness 


FIG.  99.—  Inclined  isoclinal  folds,  eroded.     (Willis.) 

in  different  parts  of  the  flexure.  This  is  especially  marked  in  fan 
folding,  in  which  the  beds  are  much  thinner  on  the  limbs  than  at 
the  summit,  and  sometimes  the  central  beds  in  the  folds  have 
been  actually  forced  to  flow  upward  or  downward,  forming  iso- 
lated masses,  cut  off  from  their  original  connections. 

Besides  the  simple  folds  above  described,  there  are  frequently 
found  complex  systems  of  flexures,  in  which  the  compressing  force 


FIG.  ioo. —  Diagram  of  monoclinal  fold. 

has  acted  simultaneously  or  successively  in  different  directions, 
producing  highly  complicated  cross-folds.     These  are,  however, 


242  DISPLACEMENTS   OF   STRATA 

vften  extremely  difficult  to  work  out,  and  in  an  elementary  book, 
intended  for  the  beginner,  it  is  not  necessary  to  do  more  than 
mention  them. 

The  monoclinal  fold  is  a  somewhat  exceptional  type,  which  can 
hardly  be  regarded  as  a  modified  form  of  the  anticline.  A  mono- 
clinal flexure  is  a  single,  sharp  bend  connecting  strata  which  lie  at 
different  levels  and  are  often  horizontal  except  along  the  line  of 
flexure.  Folds  of  this  character  are  very  common  in  many  parts 
of  the  West,  especially  in  the  high  plateau  region  of  Utah  and 
Arizona. 


CHAPTER   XIII 
DISLOCATIONS   AND  FRACTURES  OF  STRATA 

STRATA  are  often  unable  ta  accommodate  themselves  by  bend- 
ing or  plastic  flow  to  the  stresses  to  which  they  are  subjected  : 
and  instead  of  flexing  they  are  fractured,  usually  accompanied 
by  more  or  less  dislocation.  A  simple  crack  or  fracture  through 
the  strata,  not  involving  dislocation,  is  called  a  fissure.  On  the 
two  sides  of  a  fissure  the  beds  are  the  same  at  corresponding 
levels,  and  evidently  the  crack  has  been  made  through  continuous 
strata. 

Faults. — When  the  strata  on  one  side  of  a  fissure  have 
been  lifted  up  or  dropped  down,  so  that  the  strata  which  were 
once  continuous  across  the  plane  of  fracture  are  now  separated 
by  a  greater  or  less  vertical  interval,  and  lie  at  different  levels,  the 
structure  is  called  a  fault.  A  fault  is  sometimes,  but  not  very 
often,  vertical ;  usually  it  is  inclined  at  a  greater  or  less  angle, 
and  the  angle  made  by  the  intersection  of  the  fault  plane  with 
a  vertical  plane  is  called  the  hade  or  slope  of  the  fault.  The 
fault  dip  is  the  angle  included  between  the  fault  plane  and  a 
horizontal  plane,  and  is  the  complement  of  the  hade.  For 
example,  if  the  fault  is  vertical,  the  hade  is  o°  and  the  dip  90° ; 
if  the  fault  is  horizontal,  the  hade  is  90°  and  the  dip  is  o°, 
while  a  hade  of  45°  gives  a  dip  of  the  same  amount.  The  side 
on  which  the  beds  lie  -at  a  higher  level  than  their  continuations 
across  the  fracture  is  called  the  upthrow  side,  and  the  other  is 
the  downthrow  side.  Owing  to  the  obliquity  of  the  fault  plane, 
the  beds  on  one  side  usually  project  over  those  on  the  other,  and 
hence  form  the  hanging  wall;  that  side  which  extends  underneath 
the  other  is  called  the  foot  wall.  Either  the  foot  or  the  hanging 
wall  may  be  the  upthrow  or  downthrow  side,  according  to  the 

243 


244 


DISLOCATIONS   OF   STRATA 


nature  of  the  fault.  The  friction  of  the  rocks  grinding  against 
each  other  in  the  fault  frequently  smooths  and  polishes  them, 
which  gives  the  characteristic  appearance  known  as  slickensides. 

The  movements  of  the  beds  along  the  fault  plane  are  usually 
simple,  and  in  only  one  direction,  but  there  are  several  different 
kinds  of  measurements  to  be  taken,  which  express  important 
facts.  The  amount  of  vertical  displacement  between  the  two 


FIG.  loi.—  Section  through  faulted  beds,  b'  c,  throw;  b  c,  heave;  b  b' ,  strati- 
graphic  throw,  which  in  this  case  is  measured  along  the  fault  plane,  because  the 
latter  happens  to  be  at  right  angles  to  the  bedding  planes.  The  angle  b  b'  c  is  the 
angle  of  hade ;  b'  b  c,  the  angle  of  dip.  Foot  wall  to  the  right  of  the  fault,  and 
hanging  wall  to  the  left. 

fractured  ends  of  a  given  stratum  is  called  the  throw  (P  c, 
Fig.  101),  and  the  heave,  or  horizontal  throw,  is  the  horizontal 
distance  through  which  one  end  of  a  fractured  stratum  has  been 
carried  past  the  corresponding  end  on  the  other  side  of  the  fault 
plane  (b  c,  Fig.  101).  The  heave  of  a  fault  increases  in  propor- 
tion to  the  throw,  as  the  hade  increases.  In  a  vertical  fault,  which 
has  no  hade,  there  is  no  heave,  however  great  the  throw ;  for  in 
that  case  the  fractured  ends  of  the  strata  are  not  carried  past  each 
other  horizontally,  and  the  throw  of  the  fault  is  measured  along 
the  fault  plane.  With  the  amount  of  throw  remaining  constant, 


FAULTS 


245 


the  heave  increases  with  the  hade.     The  stratigraphic  throw  is 
the  thickness  of  beds  which  intervenes  between  the  broken  ends 


FIG.  102.  —  Normal  fault  of  small  throw  in  horizontal  strata :  fault  scarp  removed 
by  denudation.     (U.  S.  G.  S.) 

of  a  fractured  stratum  -(i.e.  on  opposite  sides  of  the  fault),  and  is 
measured   at  right   angles   to    the    bedding   planes.      When   the 


246  DISLOCATIONS   OF   STRATA 

faulted  strata  are  horizontal,  the  vertical  throw  is  the  same  as  the 
stratigraphic,  but  if  the  strata  are  inclined,  the  two  differ. 

Though  the  term  fault  is  applied  to  any  dislocation  of  strata, 
by  which  the  broken  ends  of  the  beds  are  carried  over  or  past 
each  other,  yet  it  includes  structures  of  very  different  significance, 
produced  in  dissimilar  ways.  Of  these  there  are  two  principal 
classes  :  — 

I.  Normal,  or  Gravity,  Faults  (Figs.  101,  102)  are  those  in 
which  the  hade  is  toward  the  downthrow,  or  depressed  side,  and 
thus  the  hanging  wall  is  on  the  latter  side,  while  the  foot  wall 
forms  the  upthrow  side.  The  phrase  "normal  fault"  is  an  unfortu- 
nate one,  but  it  seems  to  be  too  deeply  fixed  in  geological  usage 
for  any  change  to  be  practicable.  Faults  of  this  class  ordinarily 
occur  in  horizontal  or  moderately  inclined  or  folded  rocks.  As 
will  be  seen  in  a  later  section,  normal  faults  in  horizontal  or  gently 
inclined  strata  imply  the  extension  of  an  arc  of  the  earth's  crust, 
because  the  faulted  beds  occupy  more  space,  horizontally,  than 
they  did  before  dislocation.  They  are  therefore  due  to  a  tension 
greater  than  the  rocks  could  endure. 

The  normal  faults  of  any  district  may  nearly  always  be  classed 
in  two  categories  :  ( i )  Strike  faults,  which,  in  general,  follow 
the  strike  of  the  beds.  To  this  group  belong  the  great  faults,  — 
great  both  as  to  length  and  amount  of  displacement.  (2)  Dip 
faults.  These  are  more  or  less  parallel  to  the  dip  of  the  strata, 
and  therefore  form  an  open  angle  with  the  strike  faults  of  the 
same  area  :  they  are  less  important  than  the  latter. 

The  amount  of  displacement  of  faults  varies  greatly  in  different 
cases,  from  a  fraction  of  an  inch  up  to  thousands  of  feet.  In 
those  of  small  throw  the  fault  plane  is  frequently  a  clean,  sharp 
break ;  but  in  the  greater  faults  the  rocks  in  the  neighbourhood 
of  the  fault  plane  are  often  bent,  crushed,  and  broken.  The  fault 
itself  is  then  filled  up  with  a  confused  mass  of  fragments  of  all 
sizes,  which  may  be  cemented  into  a  breccia.  Such  a  mass  is 
called  fault  rock.  In  soft  rocks  the  fault  plane  is  always  closed 
by  the  immense-  weights  and  pressures  involved,  but  in  rigid 
rocks  it  may  remain  more  or  less  open,  especially  if  the  break  be 


COMPOUND   FAULTS 


247 


not  a  true  plane,  but  of  curved  and  irregular  course.  In  the 
latter  case,  there  will  be  a  succession  of  cavities  along  the  fault, 
which  frequently  are  filled  up  by  a  subsequent  deposit  of  minerals, 
and  thus  converted  into  mineral  veins. 


FIG.  103.  —  Strata  bent  upward  near  the  fault  plane.     The  hole  is  artificial. 
(U.  S.  G.  S.) 

Faults  may  die  out  in  a  few  yards,  or  they  may  run  for  hundreds 
of  miles  ;  they  may  be  simple  or  compound,  single  or  branching. 
A  compound  fault  is  made  up  of  a  number  of  parallel  dislocations, 
placed  close  together,  which  may  all  hade  in  the  same  direction, 
or  in  opposite  ways,  but  in  the  latter  case  one  hade  prevails  ovei 
the  other.  A  series  of  parallel  faults,  wider  apart  than  in  the  com- 


248  DISLOCATIONS   OF   STRATA 

pound  faults,  and  all  hading  in  the  same  direction,  are  called  step 
faults.  If  two  parallel  dislocations  hade  toward  each  other,  they 
form  a  trough  fault and  include  a  wedge-shaped  fault  block  between 
them,  which  is  on  the  downthrow  side  of  both  dislocations. 

However  long  it  may  be,  a  fault  sooner  or  later  dies  away,  the 
throw  constantly  diminishing  toward  the  ends,  until  it  vanishes. 
This  implies  that  the  fault  is  due  to  the  bending  of  the  beds  up- 
ward or  downward  along  the  plane  of  dislocation;  only  three 
intersecting  or  two  curved  faults  can  actually  isolate  a  fault  block. 
Intersecting  or  cross  fattlts  may  be  of  very  different  dates,  and 
then  the  more  ancient  ones  can  be  determined  by  the  fact  that 
they  are  themselves  dislocated. 

It  is  comparatively  seldom  that  the  upthrow  side  of  a  fault  is 
left  standing  as  a  line  of  cliffs ;  when  such  is  the  case,  the  cliffs 


'^-^"J^  ""--"- 

:^^&^^-'  /^£T~  fif^^ — t  ^=-^i     ~"'J   "•••— -r"' 

__          v     ^^-^ — *•>,.  .—  -    - jjKi'ii'.-i.-r^c ^ t-»—  — 


FIG.  104.  —  Abert  Lake,  Oregon.    The  line  of  cliffs  is  a  fault  scarp.     (Russell.) 

form  &  fault  scarp,  many  of  which  may  be  found  in  the  more  arid 
districts  of  the  West,  where  atmospheric  erosion  has  been  slow.  In 
the  great  majority  of  instances,  the  upthrow  side  suffers  more  rapid 
denudation  than  the  downthrow,  and  the  two  are  weathered  down 
to  approximately  the  same  level,  or  the  same  general  slope  (see 
Figs.  1 02,  105).  Under  these  circumstances,  faults  are  rarely  visi- 
ble on  the  surface,  and  their  presence  must  be  inferred  indirectly 
from  their  effects  upon  the  outcrops  of  the  strata  involved.  These 


STRIKE   FAULTS 


249 


effects  depend  upon  the  direction  and  throw  of  the  fault  and  upon 
the  inclination  of  the  beds.  Strike  faults  repeat  the  outcrops, 
bringing  the  beds 
again  to  the  surface ; 
in  a  series  of  paral- 
lel or  step  faults,  a 
given  stratum  has  a 
number  of  outcrops 
greater  by  one  than 
the  number  of  faults. 
Thisrepetitionofthe 
outcrop  may  be  very 
deceptive,  when  the 
surface  has  been 
planed  down  to  one 
uniform  slope  or 
curve.  In  Fig.  106, 
for  example,  the  ob- 
server might  easily 
be  misled  into  be- 
lieving that  seven 
seams  of  coal  were 
cropping  out  on  the 
hillside,  whereas  the 
section  shows  that 
there  are  only  two 
such  seams,  their 
repeated  outcrops 
being  due  to  fault- 
ing- FlG.  105.  —  Effect  of  strike  fault  on  outcrop.  A, 
The  repetitions  of  before  faulting;  B,  with  fault  scarp  standing;  C,  with 
OUtcrop, SUchas have  b°th  UPthr°W  and  downthrow  sides  denuded  to  a  con- 
tmuous  curve.  (Drawn  from  a  model  by  Sopwith.) 

been  described,  oc- 
cur when  the  throw,  or  amount  of  displacement,  is  moderate.    Great 
faults,  displaced  many  thousands  of  feet,  and  with  the  upthrow 
side  planed  away  by  denudation,  will  display  entirely  different 


25O  DISLOCATIONS   OF   STRATA 

strata  on  the  two  sides  of  the  fault.  The  deep-seated  beds,  which, 
on  the  upthrow  side,  are  exposed  by  denudation,  are,  on  the  down- 
throw side,  carried  down  so  far,  that  they  do  not  reach  the  surface 
at  all,  or,  at  least,  not  in  the  neighbourhood  of  the  fault.  Also,  if 
the  hade  of  the  fault  is  in  the  same  direction  as  the  dip  of  the 
strata,  repetition  of  outcrop  may  fail  to  occur. 


FIG.  106.  —  Effect  of  step  faults  in  repeating  outcrops.     (Drawn  from  a  model  by 

Sopwith.) 

Dip  faults  have  entirely  different  effects  upon  the  outcrops  from 
those  occasioned  by  strike  faults,  cutting  across  the  strike  of  the 
beds  and  interrupting  their  continuity.  The  outcrop  of  a  given 
stratum  ceases  abruptly  at  the  fault  line,  and  when  found  on  the 
other  side,  it  is  seen  to  be  shifted  a  greater  or  less  distance  in  the 
direction  followed  by  the  fault  plane.  How  such  a  horizontal 
shifting  is  brought  about  by  a  vertical  displacement  is  shown  by 
the  model  (Fig.  107).  In  I  the  model  is  shown  as  it  was  before 
faulting,  the  black  band  representing  a  dipping  stratum,  say  a  coal 
seam,  seen  in  section  on  the  side  of  the  model,  with  the  outcrop 
appearing  on  its  upper  surface.  In  II  the  dislocation  has  oc- 
curred, the  upthrow  side  still  standing  as  a  fault  scarp,  while  III 


DIP   FAULTS 


251 


shows  this  scarp  removed  by  denudation  and  the  two  sides  planed 
down  to  the  same  level. 
It  is  here  seen  that, 
on  the  downthrow  side, 
the  outcrop  is  shifted 
in  a  direction  opposite 
to  the  dip  of  the  beds, 
or,  what  amounts  to  the 
same  thing,  on  the  up- 
throw side,  it  is  shifted 
in  the  direction  of  the 
dip.  The  amount  of 
shifting  will  evidently 
increase  with  the  throw 
of  the  fault,  but  dimin- 
ish as  the  dip  of  the 
beds  increases. 

When  a  dip  fault 
cuts  across  eroded 
folds,  the  distance  be- 
tween the  outcrops  of 
the  same  stratum  on 
the  two  limbs  of  an 
anticline  is  increased 
on  the  upthrow  side, 
diminished  on  the 
downthrow  side ;  in 
the  synclines  this  ar- 
rangement is  reversed. 
This  is  because  on  the 
upthrow  side  the  sur- 
face of  the  ground  cuts 
the  beds  at  a  lower 
stratigraphic  level  than 
on  the  downthrow,  and 
as  the  limbs  of  an  anti- 


FlG.  107.  —  Model  showing  effect  of  dip  fault  on 
outcrop.  I,  before  faulting;  II,  with  fault  scarp 
standing;  III,  with  fault  scarp  removed  by  denuda- 
tion. 


252 


DISLOCATIONS   OF   STRATA 


cline  diverge  downward,  the  outcrops  will  be  more  widely  sepa- 
rated, the  lower  the  level  at  which  they  reach  the  surface.  In 
a  syncline  the  limbs  converge  downward,  and  the  effect  of  the 
fault  is  therefore  just  the  reverse  of  what  occurs  in  the  anticline. 


FIG.  108.  —  Great  thrust  fault,  near  Highgate  Springs,  Vt.     (U.  S.  G.  S.) 

In  an  area  traversed  by  normal  faults,  the  beds  of  the  fault 
blocks  which  are  included  between  the  parallel  or  intersecting 
faults  are  usually  not  strongly  folded.  There  is,  nevertheless,  a 
close  connection  between  faults  and  flexures,  as  the  examination 
of  any  disturbed  district  will  show.  Especially  is  this  true  of 


THRUST   FAULTS 


253 


monoclinal  folds,  which  often  pass  into  faults,  the  beds  yielding 
to  flexure  along  part  of  their  course,  fracturing  and  dislocating  in 
other  parts. 

II.  Reversed  or  Thrust  Faults  (also  called  overthrust  faults  or 
simply  thrusts). — These  are  the  opposite  of  the  sorcalled- normal 
faults,  the  hade  being  toward  the  upthrow  side,  which  thus  forms 
the  hanging  wall,  while  the  downthrow  side  is  the  foot  wall. 
Faults  of  this  class  are  due  to  compression,  the  strata  breaking 
and  slipping  past  and  over  one  another,  instead  of  folding,  and 
they  are  characteristic  of  highly  folded  regions,  sharp  plications 
often  passing  into  thrust  faults.  Thrust  faults  are  produced  in 
somewhat  different  ways,  and  have,  accordingly,  been  divided  by 
Mr.  Willis  into  four  categories. 

(1)  The  shear  thrust,  when  the  strata  are  dislocated  by  com- 
pression and  carried  along  a  fault  plane  over  other  beds ;    the 
strata  shearing  more  easily  than  bending.     Thrusts  of  this  charac- 
ter are  independent  of  flexures. 

(2)  The  break  thrust  occurs  when  the  strata  are  first  folded 
into  an  anticline  and  then  fractured,  the  upper  limb  of  the  broken 
fold  being  pushed  forward  over  the  lower. 


—  B 


FIG.  109.  —  Erosion  and  break  thrust,  Holly  Creek,  Georgia.     (Hayes.) 

(3)  The  stretch  thrust  is  caused  by  plication  and  inversion, 
by  means  of  which  the  overturned  limb  is  stretched,  broken,  and 
dislocated. 

(4)  The  erosion  thrust.     When  the  outcrop  of  a  rigid  stratum 
on  the  flank  of  an  anticline  is  caused  by  erosion,  it  will  meet  with 


254 


DISLOCATIONS   OF   STRATA 


no  resistance  when  the  compressing  force  is  again  applied,  but 
will  ride  forward  over  the  underlying  bed. 

Thrust  (or  reversed)  faults  hade  at  greater  angles  (or,  in  other 
words,  dip  at  smaller  angles)  than  do  the  normal  ones,  and  in  some 
of  the  great  thrusts  the  fault  planes  are  almost  horizontal :  thrusts 
are  always  along  the  strike,  never  parallel  with  the  dip.  If  not  of 
too  great  throw  and  heave,  thrust  faults  repeat  the  outcrop  of 
strata,  as  do  the  normal  strike  faults,  but  in  those  of  very  large 
displacement,  the  beds  which  are  brought  into  close  juxtaposition 


FlG.  no.  —  Great  thrust  fault,  near  Highgate  Springs,  Vt.  The  upthrow  side 
has  been  denuded  away  and  the  hammer  spans  the  fault,  connecting  beds  which 
are  stratigraphically  many  thousands  of  feet  apart.  (U.  S.  G.  S.) 

are  so  widely  separated  stratigraphically,  that  no  repetition  occurs. 
Faults  of  this  class  occur  on  a  grand  scale  in  the  Appalachian 
Mountains,  especially  in  the  southern  part  of  that  system,  and  even 
more  notable  examples  are  to  be  found  in  the  northwestern  part  of 
Scotland. 

THE  CAUSES  OF  FOLDING  AND  FAULTING 

The  first  step  in  this  inquiry  must  be  to  determine  the  direction 
in  which  the  folding  force  acted.  It  might  seem  natural,  at  first 


CAUSES   OF   FOLDING   AND   FAULTING  255 

sight,  to  suppose  that  the  direction  of  the  force  was  vertically  up- 
ward, acting  with  maximum  intensity  beneath  the  anticlines  and 
with  minimum  intensity  beneath  the  synclines.  But  such  an  ex- 
planation could  only  apply  to  open,  symmetrical,  and  simple  folds, 
and  even  in  these  cases,  is  not  satisfactory.  Folded  strata  must 
either  occupy  less  space  transversely  than  they  did  before  folding, 
or  else  they  must  have  been  stretched  and  made  much  thinner,  but 
a  comparison  of  continuous  beds,  in  the  flexed  and  horizontal 
parts  of  their  course,  shows  no  such  thinning.  Again,  such  an  ex- 
planation is  obviously  insufficient  to  account  for  closed,  inclined, 
and  inverted  folds,  for  contortions  and  plications,  and  for  flexures 
of  different  orders,  one  within  another. 

If  the  folding  force  did  not  act  vertically,  it  must  have  acted 
horizontally,  and  this  is  the  explanation  now  almost  universally 
accepted.  A  horizontally  acting  force  would  compress  and  crumple 
up  the  beds,  producing  different  types  of  flexure  in  accordance 
with  varying  circumstances.  Furthermore,  the  microscopic  study 
of  folded  rocks  shows  that  they  have  actually  been  compressed 
and  mashed  and  the  minutest  plications  are  visible  only  under  the 
microscope. 

Assuming,  then,  that  the  folding  force  was  one  of  compression 
and  acted  horizontally,  we  have  next  to  consider  the  circumstances 
which  modify  the  result,  producing  now  one  form  of  flexure  or 
fracture,  now  another.  Such  modifying  circumstances  are  the 
depth  to  which  a  given  stratum  is  buried,  its  thickness  and  rigidity, 
the  character  of  the  beds  which  are  above  and  below  it,  and  the 
intensity  and  rapidity  with  which  the  flexing  force  is  applied. 
When  in  a  mountain  region  one  sees  the  manner  in  which  vast 
masses  of  rigid  strata  are  folded  and  crumpled  like  so  many  sheets 
of  paper,  one  perceives  the  enormous  power  which  is  involved  in 
these  operations  and  the  gradual,  steady  way  in  which  that  power 
must  have  been  exerted.  When  strata  are  buried  under  a  suffi- 
cient depth  of  overlying  rock  to  crush  them,  they  become  virtually 
plastic  and  yield  to  the  compressing  force  by  flowing  without  fract- 
ure. At  such  relatively  great  depths  cavities  cannot  exist,  and  if 
the  compressed  rock  should  be  broken  by  the  compression,  the 


256  DISLOCATIONS   OF   STRATA 

particles  are  again  welded  together  into  a  firm  mass.  We  may 
accordingly  distinguish  a  zone  of  flow  age,  in  which  the  rocks  all 
yield  plastically,  a  more  superficial  zone  of  fracture,  in  which  all 
but  the  softest  rocks  break  on  compression,  and  between  the  two 
a  zone  of  fracture  and  flowage,  in  which  some  rocks  break  and 
others  flow,  according  to  their  rigidity.  The  depth  of  the  zone  of 
flowage  is  estimated  at  20,000  to  30,000  feet  below  the  surface. 

Strata  which  have  not  been  buried  to  a  sufficient  depth  to 
make  them  plastic,  will  yield  to  compression  by  breaking,  though 
whether  a  given  bed  is  faulted  or  flexed,  will  often  depend  upon 
whether  the  folding  force  is  applied  slowly  or  with  comparative 
rapidity.  A  force  long  acting  in  a  slow  and  steady  fashion  will 
produce  folds,  when  the  same  force  applied  more  suddenly  would 
shatter  the  beds.  Near  the  surface,  under  light  loads,  rigid  rocks 
will  always  break  rather  than  bend,  when  compressed.  Different 
stratified  rocks  differ  much  in  their  rigidity,  and  hence  a  load 
which  is  sufficient  to  cause  one  bed  to  bend  and  flow,  when  later- 
ally compressed,  will  leave  another  unaffected,  or  cause  it  to 
break,  if  the  compressing  force  overcomes  its  strength.  In  Bald 
Mountain,  New  York,  the  stiff  limestones  are  left  unchanged  by  a 
pressure  which  has  crumpled  and  contorted  the  soft  shales. 

Another  factor  of  much  importance  in  determining  the  char- 
acter and  position  of  folds  is  the  mode  in  which  the  strata  were 
originally  laid  down.  As  we  have  already  learned,  the  sheets  of 
sediment  which  cover  the  sea-bottom  are,  on  a  large  scale,  nearly 
level,  but  they  often  show  slight  departures  from  such  horizontal- 
ity  along  certain  lines.  These  initial  dips  often  determine  the 
place  of  flexures,  because  they  divert  the  thrust  from  its  horizontal 
direction. 

The  effect  exercised  by  initial  dips  is  shown  in  Fig.  112,  taken 
from  the  models  experimented  on  by  Mr.  Willis,  which,  when 
strongly  compressed,  imitate  with  remarkable  accuracy  the  struct- 
ures which  may  be  observed  in  folded  rocks.  Fig.  1 1 1  shows  that 
in  folding,  the  beds  must  slip  upon  each  other,  as  is  proved  by 
the  lines  perpendicular  to  the  bedding  planes,  which  were  contin- 
uous before  folding,  but  in  the  anticline  are  broken  by  the  differ- 


EXPERIMENTS 


257 


ential  motion  of  the  layers,  each  bed  rising  farther  up  the  slope 
than  the  one  beneath  it.     The  same  thing  must  occur  in  folded 


FlG.  in.  —  Model  showing  the  slip  of  folded  beds  upon  one  another.     (Willis.) 

rocks,  which  sometimes  show  polished  bedding  planes,  due  to  the 
slipping  of  the  beds  upon  one  another.  The  series  A  to  D  (in 
Fig.  112)  represents  a  model  before  and  in  various  stages  of 
lateral  compression, 
and  exhibits  the 
effect  of  the  slight 
initial  dip  at  x  in 
determining  the  po- 
sition of  the  anti- 
clinal fold,  which  is 
developed  by  com- 
pression. The  for- 
mation of  one  fold 
assists  in  the  devel- 
opment of  another, 
for  it  both  changes 
the  direction  of  the 
thrust  and  redistri- 
butes the  load  of 
overlying  strata. 
The  arch  of  the  an- 

.    ,.        ,.r        ,       ,       ,  FlG.  112.  —  Model  showing  effects  of  lateral  compres- 

ticlme  lifts  the  load    sion>    A  before  fo]ding>  whh  s].ght  inidal  dip  at  x .  B> 
and   diminishes   the      C,  D,  in  various  stages  of  compression.     (Willis.) 


258  DISLOCATIONS   OF   STRATA 

weight  upon  the  beds  that  lie  beneath  the  flexure,  but  increases 
the  weight  upon  the  lines  from  which  the  arch  springs. 

The  problem  regarding  the  way  in  which  this  great  lateral 
pressure  was  generated  can  best  be  considered  in  connection  with 
the  study  of  mountain  ranges. 

There  is  much  independent  evidence  to  show  that  folding  is  a 
gradual  process.  The  force  exerted  is  enormous,  but  so  is  also 
the  resistance  to  be  overcome,  and  a  steady  or  oft-renewed  com- 
pression, acting  upon  strata  under  a  great  load  of  overlying 
masses,  will  produce  regular  flexures,  where  a  sudden  compression, 
however  intense,  could  only  shatter  them. 

Thrust  or  reversed  faults  are  likewise  due  to  lateral  compres- 
sion, by  which  the  rocks  have  been  sheared  and  broken,  and  the 
beds  on  one  side  of  the  plane  of  fracture  have  been  thrust  up 
over  those  on  the  other.  A  plication  or  overturned  fold  may  often 
be  traced  into  a  thrust  fault,  in  a  way  that  shows  the  direction  of 
movement  to  have  been  the  same  in  both  fold  and  fracture. 
Numerous  experiments  also  show  that  lateral  compression  will 
produce  just  such  faults.  A  reduction  of  the  overlying  load,  by 
diminishing  the  plasticity  of  the  rocks,  will  occasion  shearing  and 
overthrusts,  when,  under  a  greater  load,  the  same  strata,  exposed 
to  an  equal  force  of  compression,  will  simply  flex  and  bend.  As 
we  have  seen  (see  Fig.  109),  an  anticlinal  fold  whose  load  has  been 
reduced  by  erosion,  will,  on  renewed  compression,  fracture  and 
develop  a  thrust  fault.  • 

Normal  or  gravity  faults  are  due  to  tension,  because,  except  in 
the  rare  case  of  those  with  vertical  hade,  the  faulted  strata  occupy 
more  space  transversely  than  they  did  before  faulting  took  place. 
It  is  not  easy  to  explain  how  this  tension  has  been  generated.  In 
part,  it  seems  to  be  due  to  a  sinking  of  the  downthrow  side, 
through  the  action  of  gravity,  and,  in  part,  to  the  application  of  a 
compressing  force  acting  at  right  angles  to  that  which  produced 
the  folds,  thus  raising  the  upthrow  side  into  a  very  broad  and 
gentle  arch,  which  is  parallel  to  the  plane  of  f ; tilting.  Faults  of 
this  class  are  intimately  connected  with  monoclinal  folds,  into  which 
they  sometimes  pass,  as  in  the  plateau  region  of  Arizona  and  Utah. 


DIMINUTION   OF   FAULTS  259 

Faults  must  be  comparatively  superficial  phenomena,  because 
under  sufficient  load  any  rock,  however  rigid,  will  behave  like  a 
plastic  body,  and  will  adjust  itself  to  stress  by  flovvage.  If  this 
be  true,  faults  must  diminish  downward,  passing  probably  into 
flexures  below.  Faults  may  also  diminish  and  die  away  upward, 
when  the  fractured  beds  are  much  more  brittle  than  those  which 
overlie  them. 


CHAPTER    XIV 

CLEAVAGE,   JOINTS,   MINERAL  VEINS,   UNCONFORMITY 

Cleavage,  Fissility,  and  Schistosity.  —  Cleavage  is  "  a  capacity 
present  in  some  rocks  to  break  in  certain  directions  more  easily 
than  in  others/'  vjtifa  fissility  is  a  "  structure  in  some  rocks,  by  virtue 


FlG.  113.  —  Fissile  quartzite,  California.     (U.  S.  G.  S.) 

of  which  they  are  already  separated  into  parallel  laminae  in  a  state 
of  nature.  The  term  fissility  thus  complements  cleavage,  and  the 
two  are  included  under  cleavage  as  ordinarily  defined  "  (Van  Hise). 
Schistosity,  or  foliation,  is  either  cleavage  or  fissility,  or  both, 

260 


CAUSE  OF  CLEAVAGE 


26l 


causing  the  rock  to  part  into  plates  with  a  rough  or  undulating 
surface,  due  to  the  presence  of  parallel  flakes  of  some  mineral. 

Many  unmodified  igneous  rocks  have  a  marked  cleavage,  which 
is  occasioned  by  the  arrangement  of  the  mineral  grains  with  their 
long  axes  parallel,  or  by  a  parallelism  in  the  cleavage  planes  of 
these  minerals,  or  by  both  factors  combined.  In  cleaved  sedi- 
mentary rocks  the  cleavage  planes  may  coincide  with  the  planes 
of  stratification.  Much  more  commonly,  however,  they  intersect 
the  latter  at  all  possible  angles,  keeping  a  constant  direction  for 
long  distances  (parallel  to  the  axes  of  the  folds  in  which  they 
occur),  while  the  bedding  planes  change  with  the  dip  from  point 
to  point.  Ordinary  roofing  slate  is  one  of  the  best  possible  ex- 
amples of  a  cleaved  rock,  and  the  structure  is  often  called  slaty 
cleavage,  to  distinguish  it  from  mineral  cleavage  (see  p.  13). 

Cause  of  Cleavage  and  Fissility.  —  It  is  very  generally  agreed 
among  geologists  that  slaty  cleavage  is  a  result  of  compression ; 
for,  disregarding  certain  igneous  masses,  it  occurs  only  in  rocks 
which  show  other 
evidences  of  having 
been  subjected  to 
compression.  On 
the  other  hand,  the 
mechanics  of  the 
problem  are  some- 
what obscure  and 
have  given  rise  to 
differences  of  opin- 
ion. The  most  prob- 
able view  seems  to  be  that  the  cleavage  planes  are  developed 
at  right  angles  to  the  compressing  force,  and  are  due  to  the 
arrangement  of  the  constituent  mineral  particles  of  the  rock 
with  their  longest  diameters,  their  cleavage  planes,  or  both,  in 
parallel  directions.  Further,  that  "  this  arrangement  is  caused, 
first  and  most  important,  by  parallel  development  of  new  minerals  ; 
second,  by  the  flattening  and  parallel  rotation  of  old  and  new  min- 
eral particles ;  and  third,  and  of  least  importance,  by  the  rotation 


FIG.  114. —  Diagram  showing  relation  of  cleavage  and 
stratification  planes. 


262  JOINTS 

into  approximately  parallel  positions  of  random  original  particles  " 
(Van  Hise).  Cleavage  is  developed  by  a  compressive  force  at 
depths  where  the  rocks  are  under  sufficient  weight  of  overlying 
masses  to  be  plastic,  and  therefore  in  the  zone  of  flowage. 

Fissility  is  likewise  due  to  compression,  but  in  this  case,  the 
rocks  yield  along  the  shearing  planes ,  which  are  inclined  and  not 
normal  to  the  direction  of  pressure.  Fissility  arises  in  the  zone 
of  fracture  where  the  rocks  are  not  loaded  heavily  enough  to  be 
plastic ;  but  as  the  depth  of  this  zone  varies  for  rocks  of  different 
degrees  of  rigidity,  cleavage  may  be  produced  in  a  softer  stratum 
and  fissility  in  a  harder  one  at  the  same  depth. 

Joints.  —  With  the  exception  of  loose  incoherent  masses,  like 
sand  and  clay,  all  rocks,  however  they  may  have  been  formed,  are 
divided  into  blocks  of  greater  or  less  size  by  systems  of  cracks, 
known  as  joints.  These  may  be  well  observed  in  any  stone-quarry, 
because  the  art  of  quarrying  consists  in  utilizing  these  natural 
divisions  of  the  rock.  Were  it  not  for  them,  quarrying  would  be 
much  more  difficult  and  costly  than  it  is. 

In  the  igneous  rocks  all  the  division  planes  which  separate  the 
mass  into  blocks  are  true  joints,  and  they  vary  much  in  the  way 
in  which  they  intersect  one  another,  and  in  the  consequent  shape 
of  the  blocks.  The  fine-grained  basaltic  rocks  display  a  very  gen- 
eral tendency  to  columnar  jointing,  forming  more  or  less  regular 
prismatic  columns,  which  are  more  frequently  hexagonal  than  of 
other  shapes.  In  our  study  of  modern  volcanoes  (p.  48)  we 
learned  that  certain  modern  lavas  display  these  same  characteristic 
hexagonal  columns  and  the  ancient  basalts  of  very  many  regions 
have  them.  In  certain  cases,  as  in  the  famous  Giant's  Causeway, 
of  Ireland,  the  columns  are  divided  transversely  by  concave  joints, 
giving  a  curious  "  ball  and  socket "  structure,  which  almost  seems 
artificial.  While  regular  hexagonal  columns  are  much  more  fre- 
quent in  the  fine-grained  basalts,  they  are  not  confined  to  that 
family.  The  acid  glass  of  Obsidian  Cliff,  in  the  Yellowstone  Park, 
is  jointed  in  the  same  fashion,  though  somewhat  less  regularly 
(Fig.  15).  Mato  Tepee  of  South  Dakota  is  a  mass  of  phonolite, 
jointed  into  magnificent  columns  (see  Fig.  127),  and  other  exam- 


MASTER   JOINTS  263 

pies  might  be  mentioned.  In  many  of  the  granites  and  other 
coarse-grained  igneous  rocks,  the  joints  are  so  arranged  as  to 
divide  the  mass  into  cubical  blocks,  or  into  long,  rectangular 
prisms.  In  others,  again,  the  blocks  are  of  exceedingly  irregular 
form  and  size. 

In  sedimentary  rocks  the  joints  are  ordinarily  in  only  two  planes, 
the  third  being  given  by  the  bedding  planes.     In  homogeneous, 


FIG.  115.  —  Slip  Rock,  Juniata  River,  Pennsylvania.   Steeply  inclined  strata,  showing 
joints.     (Photograph  by  Rau.) 

heavily  bedded  sediments,  such  as  limestones  and  massive  sand- 
stones, the  joints  are  apt  to  form  cubical  or  rectangular-prismatic 
blocks,  making  a  weathered  cliff  look  like  a  gigantic  wall  of  regular 
masonry.  Other  sedimentary  rocks  are,  as  a  rule,  more  irregularly 
jointed. 

Joints  are  of  very  different  orders  of  importance  :  some,  the 
master  joints,  traverse  many  strata  and  remain  constant  for  long 
distances  and  considerable  depths,  while  each  layer  usually  has 


264  JOINTS 

minor  joints  which  are  confined  to  that  bed.  One  set  of  joints, 
the  strike  joints,  run  more  or  less  parallel  to  the  strike  of  the 
beds,  while  the  second  set,  the  dip  joints,  follow  the  dip ;  the 
former  are  usually  the  longer  and  more  conspicuous. 

Cause  of  Joints.  —  With  regard  to  the  manner  of  their  produc- 
tion, joints  maybe  classified  into  two  series  :  (i)  those  which  are 
due  to  tension,  the  rock  usually  parting  in  planes  normal  to  the 
directions  of  tension;  (2)  those  which  are  due  to  compression, 
the  cracks  forming  in  the  shearing  planes. 

( i )  Tension  Joints.  —  In  igneous  rocks  joints  are  caused  by  the 
cooling  and  consequent  contraction  of  the  highly  heated  mass.  This 
shrinkage  sets  up  tensile  stresses  in  the  mass  to  which  the  rock  yields 
by  cracking  and  parting,  the  shape  of  the  blocks  being  largely  con- 
trolled by  the  coarseness  or  fineness  of  the  mass.  In  some  cases  the 
jointing  of  sedimentary  rocks  may  perhaps  be  caused  by  a  shrinkage 
of  the  mass  on  drying,  but  this  cannot  be  an  important  method  of 
producing  systems  of  joints. 

The  convex  sides  of  anticlinal  and  synclinal  folds  are  stretched, 
and  (provided  they  are  not  too  deeply  buried)  the  stretching  may 
result  in  a  system  of  cracks  radial  to  the  curves  which  follow  the 
strike  of  the  beds.  Folds  are  not  horizontal,  but  pitch  in  the 
direction  of  their  axes.  This  complex  folding  may  produce  two 
sets  of  tensile  stresses  perpendicular  to  each  other,  and  thus  cause 
two  series  of  joints,  one  following  the  strike  and  the  other  the  dip 
of  the  beds.  Complex  folding  must  produce  a  twisting  and  warp- 
ing of  the  strata,  and  it  has  been  experimentally  shown  that  a 
brittle  substance  when  twisted  cracks  in  two  sets  of  fractures  which 
intersect  nearly  at  right  angles.  How  slight  is  the  twisting  and 
warping  needful  to  produce  joints  is  shown  by  the  fact  that  strata 
which  are  perfectly  horizontal,  so  far  as  can  be  detected,  are 
jointed,  as  are  also  certain  modern  coral  limestones. 

Tension  joints  produce  either  rough,  or  smooth  and  sharply 
cut  surfaces,  which  is  determined  by  the  character  of  the  rock. 
In  sandstones  which  are  weakly  cemented  the  cracks  pass 
between  the  grains,  while  in  hard  and  firm  rocks  the  fractures 
are  clean. 


MINERAL  VEINS  265 

(2)  Compression  Joints  are  caused  when  the  rocks  yield  along 
the  shearing  planes.  In  simply  folded  strata  are  produced  two  sets 
of  strike  joints  which  are  inclined  toward  each  other,  but  whether  dip 
joints  will  be  made  by  complex  folding  is  not  certain.  In  certain 
conglomerates  the  joint  planes  pass  through  the  hard  quartz  pebbles 
and  leave  a  smooth,  even,  shining  face.  Tension  would  pull  such 
a  pebble  out  of  its  socket  and  only  by  shearing  could  it  be  cleanly 
cut.  Compression  joints  are  merely  a  special  case  of  fissility.  If 
the  division  planes  be  many  and  close  together,  they  constitute 
fissility  ;  if  more  widely  spaced,  jointing. 

The  whole  subject  of  joints  in  sedimentary  rocks  is  a  difficult 
one  and  the  explanations  given  of  them  are  not  altogether  satisfac- 
tory, for  several  other  agencies  may  be  involved  in  their  produc- 
tion. It  is,  however,  highly  probable  that  the  master  joints  which 
roughly  follow  the  strike  and  dip  of  the  strata,  have  been  caused 
by  the  forces  which  produce  folding. 

Joints  cannot  occur  in  the  zone  of  flowage,  and  are  best  devel- 
oped in  the  zone  of  fracture,  being  of  less  importance  in  the  transi- 
tion belt  between  the  two. 

Folds  and  faults,  cleavage,  fissility,  and  joints  may  all  be  re- 
garded as  the  varying  products  of  the  same  set  of  forces,  lateral 
compression  and  gravity.  Just  what  type  of  structure  is  to  result 
depends  upon  the  circumstances  under  which  the  forces  are  ap- 
plied, the  nature  of  the  rocks  affected,  the  depths  to  which  they 
are  buried,  etc.  Joints  and  fissility  are  minute,  incipient  faults, 
and  whether  a  rock  is  flexed  or  faulted  is  determined  by  its  rigid- 
ity, the  load  which  it  carries,  and  the  gradual  or  sudden  applica- 
tion of  the  lateral  compression. 

MINERAL  VEINS 

The  gaping  faults  and  fissures  which  traverse  hard  rocks  gener- 
ally remain  more  or  less  open  for  a  time,  and  are  frequently  filled 
up  by  a  subsequent  deposition  of  material,  quite  different;  from 
the  rock  which  forms  the  walls  of  the  fissure  or  fault  and  which  is 
called  the  "  country  rock."  Fissures  thus  filled  by  crystallized 
deposits  are  called  mineral  veins,  which  may  be  either  simple  or 


266  MINERAL   VEINS 

banded.  In  the  former  case  the  vein  is  filled  with  a  single  min- 
eral, while  in  the  latter  mineral  substances  are  arranged  in  bands, 
which  are,  in  general,  parallel  to  the  walls  of  the  fissure.  These 
bands  were  evidently  deposited  on  the  walls  of  the  fissure,  for  the 
more  perfect  ends  of  the  crystals  project  inward  toward  the  mid- 
dle line  of  the  vein,  and  the  bands  are  arranged  in  corresponding 
pairs,  from  the  walls  inwards.  In  many  instances  an  apparent 
departure  from  this  symmetrical  arrangement  is  produced  by  a 
reopening  of  the  fissure,  so  that  the  older  vein  forms  one  of  the 
walls  of  the  new  one.  Frequently  the  minerals  are  associated  with 
an  eruption  of  igneous  rock,  and  the  minerals  are  deposited  along 
the  plane  of  contact  between  the  igneous  mass  and  the  country 
rock. 

Often  mineral  veins  contain  in  greater  or  less  richness  the  ores 
of  various  metals,  and  then  they  are  called  metalliferous  veins. 
From  such  are  obtained  the  greater  part  of  the  world's  supply 
of  gold,  silver,  copper,  tin,  etc.  The  minerals  which  form  the 
larger  bulk  of  the  vein  are  called  vein  sttiff,  or  gangue,  and  the  ores 
are  either  gathered  in  threads,  pockets,  or  nuggets,  or  dissemi- 
nated in  fine  grains  throughout  the  mass  of  the  vein  stuff.  The 
principal  minerals  which  make  up  the  latter  are  quartz,  calcite, 
barite  (heavy  spar,  BaSO4),  and  fluor-spar,  while  the  metals  are 
sometimes  native  (i.e.  in  the  free,  uncompounded  state),  as  are 
gold  and  platinum  nearly  always,  and  copper  frequently,  but  they 
much  more  commonly  occur  as  sulphides,  chlorides,  oxides,  car- 
bonates, or  other  combination. 

The  outcrop  of  a  mineral  vein  at  the  surface  of  the  ground  is 
much  altered  by  weathering  and  the  true  character  of  the  vein 
may  appear  only  after  it  has  been  worked.  The  chemical  changes 
produced  by  weathering  vary,  of  course,  with  the  nature  of  the 
materials  acted  on,  and  a  single  example  must  suffice  here.  In 
the  deeper,  unaltered  portions  of  many  gold-bearing  veins  the 
gold  is  contained  in  crystals  of  iron  pyrites.  For  a  varying  depth 
below  the  surface,  the  gold  is  native  and  is  scattered  in  minute 
threads  and  grains  through  a  mass  of  broken  quartz,  stained  rusty 
red  or  brown  by  iron,  while  the  pyrite  has  disappeared.  The 


FORMATION   OF  VEINS  267 

change  has  been  brought  about  in  the  following  way.  Pyrite,  on 
exposure  to  air  and  moisture,  slowly  absorbs  oxygen  and  is  con- 
verted into  the  sulphate  of  iron  (FeSo4) .  The  latter  is  an  unstable 
compound,  and  continued  exposure  to  air  and  water  converts  it 
into  limonite,  or  ferric  hydrate,  with  liberation  of  sulphuric  acid ; 
the  gold  is  thus  left  free  and  the  limonite  stains  the  broken  quartz 
rusty  brown  or  red. 

As  mineral  veins  so  frequently  occupy  faults,  they  generally  are 
inclined  at  high  angles,  and  like  faults  may  be  intersected  by  other 
veins.  If  the  vein  thus  cut  have  any  hade,  or  inclination  from  the 
vertical,  it  will  itself  be  dislocated  and  slipped,  just  like  an  ordi- 
nary stratum.  In  this  way  we  may  determine  which  of  two  cross 
veins  is  the  older,  for  it  will  be  faulted,  while  the  newer  one  will 
continue  uninterruptedly  across  the  line  of  intersection. 

When  we  attempt  to  determine  the  manner  in  which  mineral 
veins,  and  especially  the  metalliferous  varieties,  have  been  formed, 
we  find  many  facts  that  are  extremely  puzzling,  and  no  explana- 
tion has  been  devised  that  will  cover  all  cases.  The  great  economic 
importance  of  metalliferous  veins  has  caused  them  to  be  carefully 
studied  in  many  different  parts  of  the  world,  but  experience  gained 
in  one  region  is  apt  to  be  contradicted,  in  one  or  more  particu- 
lars, by  that  gathered  in  other  regions.  In  a  general  way,  we  may 
be  confident  that  the  minerals  were  deposited  from  hot  alkaline 
solutions,  brought  up  from  the  depths  of  the  fissure,  just  as  we 
have  found  to  be  the  case  in  certain  hot  springs  now  active  (see 
p.  130).  The  vein  stuffs  show,  both  by  their  arrangement  in  the 
fissure  and  by  their  microscopic  characters,  that  they  have  been 
deposited  from  solution,  and  such  minerals  as  quartz  are  dissolved 
in  quantity  only  by  hot  alkaline  waters.  Both  from  observation 
and  experiment  we  learn  that  the  alkaline  sulphides  are  the  natural 
solvents  of  the  heavy  metallic  sulphides,  the  form  in  which  so 
many  of  the  ores  occur.  The  materials  deposited  appear  to  be 
derived  from  various  depths  of  the  fissure,  and  it  has  been  noticed 
in  many  instances  that  the  contents  of  the  vein  change,  as  the 
country  rock  changes. 

Lead  deposits  occur  in  a  different  fashion  from  the  metals  men- 


268  SEDIMENT-FILLED   VEINS 

tioned  above.  In  the  Mississippi  valley,  from  Wisconsin  to 
Arkansas,  are  found  great  quantities  of  galena,  the  sulphide  of 
lead  (PbS),  deposited  in  cavities  of  limestone;  the  cavities  follow 
the  lines  of  joint  and  are  largest  along  the  master  joints.  Associ- 
ated with  the  lead  frequently  occur  ores  of  zinc,  pyrite,  marcasite, 
and  other  minerals.  How  these  remarkable  accumulations  were 
formed  is  altogether  doubtful. 


FlG.  116.  —  Dikes  of  sandstone  in  shales.     Northern  California.     (U.  S.  G.  S.) 

Sediment-filled  Veins. — Vertical  fissures  are  sometimes  filled 
up  by  sediment  washed  in  from  above,  but  more  remarkable  are 
the  instances  where  the  fissure  evidently  did  not  communicate 
with  the  surface  and  yet  was  filled  with  sediment  different  from 
the  walls.  In  Fig.  116  is  seen  an  example  from  northern  Cali- 
fornia :  the  fissures  which  traverse  the  shale  have  been  filled  with 


UNCONFORMITY  269 

sand,  which,  consolidating,  has  resisted  weathering  better  than  the 
shale  and  so  stands  out  in  relief.  These  are  called  sandstone 
dikes,  though  they  are  not  true  dikes,  which  are  of  igneous  rock. 
The  sand,  in  this  and  similar  cases,  has  been  forced  in  by  pressure 
from  below  while  still  loose. 

UNCONFORMITY 

We  have  hitherto  considered  the  stratified  rocks  as  made  up  of 
beds  which  follow  upon  one  another  in  orderly  sequence,  and  are 
all  affected  alike  by  the  elevation  or  depression,  folding  or  disloca- 
tion, to  which  they  may  have  been  subjected.  Strata  which  have 
thus  been  laid  down  in  uninterrupted  succession,  with  sensibly 
parallel  bedding  planes,  and  which  have  been  similarly  affected  by 
movements,  are  said  to  be  conformable,  and  the  structure  is  called 
conformity.  In  many  places,  however,  the  strata  exposed  in  a 
section  are  very  obviously  divisible  into  two  groups,  each  made  up 
of  a  series  of  conformable  beds,  but  the  upper  group,  as  a  whole, 
is  not  conformable  with  the  lower,  but  rests  upon  its  upturned 
edges,  or  its  eroded  surface.  The  two  groups  are  said  to  be 
unconformable  and  the  structure  is  named  unconformity.  The 
definition  of  unconformity  here  given  includes  certain  not  uncom- 
mon structures,  which  must  be  distinguished  as  having  quite  a 
different  significance. 

Unconformity  is  of  two  kinds.  ( i )  There  is  a  distinct  difference 
in  the  dip  of  the  two  sets  of  strata,  the  upper  beds  lying  across  the 
upturned  and  truncated  edges  of  the  lower.  This  is  the  more 
usual  kind  and  is  shown  in  Figs.  117  and  133.  The  structure  im- 
plies that  the  lower  series  of  beds  was  first  laid  down  under  water, 
and  that  they  were  then  upturned,  tilted  or  folded  to  form  a  land 
surface.  Erosion  next  truncated  the  folds,  planing  the  edges  of 
the  disturbed  beds  down  to  a  more  or  less  level  surface.  The 
land  surface  was  again  depressed  beneath  the  water,  and  the 
second  set  of  strata  was  deposited  upon  it.  Finally,  a  renewed 
elevation,  accompanied  perhaps  with  folding  or  faulting,  has 
brought  both  series  of  strata  above  the  sea-level. 

While  the  older  beds  formed  a  land  surface,  they  were  eroded 


2/0 


UNCONFORMITY 


and  no  deposition  took  place  upon  them.  Consequently,  between 
the  two  sets  of  strata  is  a  gap,  unrecorded  by  sedimentation  (at 
that  point),  the  length  of  which  represents  the  time  that  the  older 
beds  were  above  water.  The  processes  involved  in  an  unconform- 
ity are  of  slow  operation,  so  that  the  gap  usually  implies  a  very 
long  lapse  of  time.  In  many  cases  whole  geological  ages,  of  in- 


FlG.  117.  —  Unconformity. 


Diagrammatic  section  through  the  strata  seen  in 
Fig.  133,  P-  3J7. 


calculable  duration,  have  intervened  between  the  deposition  of  the 
two  groups  of  strata. 

(2)  In  the  second  kind  of  unconformity  the  two  groups  of 
strata  have  the  same  dip,  the  upper  series  resting  upon  the  eroded 
surfaces  of  the  lower.  The  processes  involved  in  this  kind  of  un- 
conformity are  nearly  the  same  as  in  the  first,  so  far,  at  least,  as  the 
alternation  of  land  surface  and  sea-bottom,  elevation  and  depres- 
sion, are  concerned.  In  this  case,  however,  the  first  upheaval  was 
not  accompanied  by  any  folding  or  fracturing  of  the  beds.  An 
unconformity  of  the  second  class  is  sometimes  exceedingly  diffi- 
cult; to  detect  a,ncl  then  is  called  a  deceptive  conformity.  Such  a 


OVERLAP 


271 


case  arises  when  the  surface  of  the  ground  is  made  by  cutting 
down  strata  to  the  upper  surface  of  a  hard  bed,  which  is  then  de- 
pressed beneath  the  water,  as  a  flat  pavement,  upon  which  new 
material  of  a  similar  kind  is  laid  down  with  hardly  a  perceptible 
break.  In  the  Rocky  Mountain  region  remarkable  instances  of 
this  deceptive  conformity  occur,  where,  in  the  middle  of  a  mass  of 
limestone  apparently  formed  without  any  interruption,  there  is,  in 
reality,  an  enormous  time-gap.  Long  and  careful  search  has 
made  clear  the  nature  of  the  contact  and  exposed  the  deception. 
The  lowest  member  of  the  upper  series  of  strata  in  an  uncon- 
formity is  very  frequently  a  conglomerate  or  coarse  sandstone,  and 
represents  the  beach  formation  of  the  sea  advancing  over  the  old 


FIG.  118.  —  Unconformity  without  change  of  dip,  and  overlap. 

land.  These  are  called  basal  conglomerates.  Such  coarse  beds 
are,  however,  not  always  present,  and  they  may  be  only  locally 
developed  along  a  particular  line. 

Unconformities  may  be  confined  to  relatively  restricted  regions, 
or  they  may  extend  over  whole  continents ;  they  are  very  useful 
means  of  dividing  the  strata  into  natural  chronological  groups. 

Overlap.  — When  a  series  of  strata  is  deposited  in  a  basin  with 
sloping  sides,  or  on  one  sloping  side,  each  bed  will  extend  farther 
than  the  one  upon  which  it  lies,  and  thus  in  a  thick  mass  of  strata, 
if  the  shelving  bottom  be  gently  inclined,  the  upper  beds  will  ex- 
tend far  beyond  the  lower  ones,  or  overlap  them.  (See  Fig.  118.) 
Overlap  also  occurs  where  the  sea  is  advancing  or  transgressing 
slowly  across  a  subsiding  land  surface,  the  rate  of  depression  not 
much  exceeding  the  rate  of  deposition.  Here  also  each  stratum 


272 


CONTEMPORANEOUS    EROSION 


extends  farther  across  the  old  land  surface  than  the  one  beneath 
it,  and  conceals  the  edges  of  the  latter.  The  relation  of  overlap 
is  between  the  successive  layers  of  a  conformable  series. 

Overlap  may  be  a  structure  of  much  economic  importance, 
if  one  of  the  lower  strata,  say  a  coal-bed,  is  mined.  It  is  not 
safe  to  assume  that  wherever  the  upper  beds  of  such  a  series  are 
found,  the  lower  will  be  found  directly  beneath  them,  an  assump- 
tion which  may  result  in  costly  failure. 


FIG.  119.  —  Contemporaneous  erosion  in  limestone,  Iowa.      (Photograph  by  the 
Iowa  Geological  Survey.) 

Contemporaneous  Erosion.  —  It  was  stated  above  that  the  defi- 
nition of  unconformity,  as  given,  would  include  certain  structures, 
which,  nevertheless,  must  be  distinguished  from  it :  one  of  these 
is  contemporaneous  erosion.  This  structure  is  produced  when  a 
current  of  water  excavates  channels  for  itself  in  the  still  soft  and 
submerged  mass  of  sediment.  After  the  current  has  ceased  to 
flow,  renewed  deposition  fills  up  the  hollow  with  the  same  or  a 


OBLIQUE   BEDDING  2/3 

different  kind  of  material  as  was  thrown  down  before.  This 
structure  requires  only  a  short  pause  in  deposition,  not  a  long, 
unrecorded  break,  and  does  not  necessarily  involve  movements 
of  elevation  and  depression.  Furthermore,  contemporaneous 
erosion  is  a  local  phenomenon,  and  though  in  a  limited  section 
it  may  not  always  be  easy  to  distinguish  it  from  an  uncon- 
formity, the  difference  becomes  apparent  when  a  wider  area  is 
examined.  If  the  structure  be  one  of  contemporaneous  erosion, 
the  two  series  of  strata  will  be  conformable  except  along  the  line 
of  the  channel  or  channels.  In  Fig.  119  is  an  example  of  this 
structure  and  shows  where  a  channel  in  an  ancient  sea-bottom  of 
calcareous  material  was  filled  up  by  a  later  deposition  of  similar 
substance. 

The  clay  "horses"  (as  miners  call  them),  which  frequently 
interrupt  coal  beds,  are  the  channels  of  streams  which  meandered 
through  the  ancient  peat  bog,  and  which  were  filled  up  with  sedi- 
ment when  the  swamp  became  submerged.  The  "horses"  are 
usually  of  the  same  rock  as  that  which  forms  the  cap  or  roof 
of  the  coal  seam. 

Horizontal  and  Oblique  Bedding.  —  Another  kind  of  deceptive 
resemblance  to  unconformity  is  occasionally  caused  by  the  alter- 
nation of  horizontal  and  oblique  bedding,  a  horizontal  bed  resting 
upon  a  series  of  inclined  layers.  A  conspicuous  example  of  this 
is  given  by  the  Le  Clair  limestone  of  Iowa,  which  was  at  one  time 
altogether  misunderstood,  but  the  deception  is  seldom  one  that  a 
little  care  will  not  expose.  (See  also  Fig.  77.) 
T 


CHAPTER    XV 
UNSTRATIFIED  OR  MASSIVE  ROCKS 

THE  unstratified  or  massive  rocks  have  risen  in  a  molten  state 
from  below  toward  the  surface,  though  by  no  means  always  reach- 
ing it,  and  have  forced  their  way  through  or  between  the  stratified 
rocks.  One  of  the  most  important  points  to  determine  with 
regard  to  a  massive  rock  is  its  relation  to  the  strata  in  which  it 
occurs ;  for  the  earth's  chronology  is  given  by  the  stratified  rocks. 
Considered  only  with  reference  to  itself,  an  igneous  mass  gives  no 
trustworthy  evidence  as  to  the  time  when  it  was  formed.  The 
term  eruptive  is  frequently  employed  in  the  same  sense  as  unstrati- 
fied, because  of  the  belief  that  most  igneous  masses  have  been 
connected  with  volcanoes;  but  as  such  a  belief  may  not  be  well 
founded,  it  is  better  to  use  a  non-committal  term. 

We  shall  first  take  up  the  volcanic  rocks,  because  modern 
volcanoes  give  us  the  key  by  which  we  may  readily  interpret 
them. 

I.   ANCIENT  VOLCANOES  AND  THEIR  ROCKS 

Volcanic  Necks.  —  Volcanoes,  like  all  other  mountains,  are  sub- 
ject to  the  destructive  effects  of  the  atmosphere,  rivers,  and  the 
sea.  In  an  active  volcano  the  upbuilding  by  lava  flows  and  frag- 
mental  ejections  more  than  compensates  for  the  loss  by  weather- 
ing, and  the  cone  continues  to  grow  in  height  and  diameter. 
When  the  volcano  has  become  extinct,  the  destructive  agencies 
work  unopposed.  We  find  extinct  volcanoes  in  all  stages  of 
degradation,  from  those  which  look  as  though  their  activity 
might  be  renewed  at  any  moment,  to  those  which  require  the 
careful  examination  of  a  skilled  geologist  to  recognize  them  for 
what  they  are. 

274 


VOLCANIC  NECKS 


275 


In  the  Pacific  States  may  be  found  admirable  examples  of 
volcanic  cones  in  various  stages  of  erosion.  In  northern  Arizona 
the  picturesque  San  Francisco  mountains,  themselves  volcanic,  are 
surrounded  by  numerous  small  and  very  perfect  cones,  hardly 
affected  by  weathering.  In  northern  California  stands  the  noble 
peak  of  Mt.  Shasta  (Fig.  18),  which  was  active  till  a  late  geologi- 
cal date  and  still  shows  traces  of  activity  in  its  hot  vapours,  but 
has  begun  to  suffer  notably  from  weathering.  Still  farther  north, 
in  the  State  of  Washington,  is  Mt.  Rainier,  another  volcanic  cone, 
which  has  been  longer  exposed  to  the  destructive  agencies  and 
has  been  worn  into  an  exceedingly  rugged  peak. 


FIG.  120.  — Volcanic  neck,  New  Mexico.     (U.  S.  G.  S.) 

These  mountains,  however,  merely  exemplify  the  earliest  stages  of 
degradation  ;  as  time  goes  on,  the  loftiest  cones  will  be  worn  away, 
all  the  more  rapidly,  if  they  be  composed  principally  of  fragmental 
materials.  At  last  only  the  worn-down  and  hardly  recognizable 
stump  of  the  volcano  remains,  which  is  known  as  a  volcanic  neck. 
The  neck  consists  of  the  funnel  or  vent  filled  up  with  the  hard- 
ened lava  of  the  last  eruption,  or,  less  commonly,  with  a  mass  of  vol- 


2/6 


UNSTRATIFIED    ROCKS 


canic  blocks.  Associated  with  this  plug  of  lava  may  be  preserved 
the  lowest  lava  flows  or  tuffs  of  which  the  cone  was  originally 
built  up.  If  the  land  upon  which  the  volcanic  neck  stands  be 
covered  by  the  sea  or  other  body  of  water,  the  remnant  of  the 


FIG.  121. — Jointed  lava  flow.     Passaic  River,  New  Jersey.     (U.  S.  G.  S.) 

cone  will  be  buried  beneath  sediments,  and  a  volcanic  island 
may  be  similarly  cut  down  and  covered  with  sediments.  Subse- 
quent upheaval  and  denudation  may  at  a  long  subsequent  time 
once  more  expose  the  buried  cone  to  view.  Several  examples  of 
this  have  been  found  in  Great  Britain. 


INTRUSIVE   MASSES 

Lava  Flows  and  Sheets  which  were  poured  out  on  the  surface 
of  the  ground  may  be  recognized  by  the  aid  of  several  criteria. 
In  flows  of  only  moderate  antiquity,  which  have  suffered  little 
denudation,  the  nature  of  the  mass  may  be  determined  at  a  glance, 
and  traced  to  the  vent  whence  it  issued.  Successive  sheets,  piled 
one  over  the  other  in  a  rude  bedding,  are  also  evidence  that  the 
rocks  are  surface  lavas.  (See  Fig.  21,  p.  57.)  Surface  sheets  may 
be  overlaid  by  sediments,  deposited  upon  a  submarine  flow,  or 
after  depression  of  the  land.  Such  a  flow  is  then  called  a  con- 
temporaneous or  interbedded  sheet,  and  evidently  its  geological  age 
follows  the  rule  for  strata ;  it  is  newer  than  the  bed  upon  which 
it  lies  and  older  than  the  one  which  rests  upon  it. 

Fragmental  Products  (Pyroclastic)  are  positive  proof  of  vol- 
canic action,  for  they  cannot  be  formed  underground.  Coarse 
masses  of  agglomerate,  blocks,  and  bombs  show  that  the  vent 
from  which  they  issued  was  not  far  away,  while  beds  of  fine  ashes 
and  tuffs  may  be  made  at  great  distances  from  their  source.  All 
these  varieties  may  be  enclosed  in  true  sediments,  and  may,  in  part, 
escape  destruction  long  after  the  volcano  which  ejected  them  has 
been  cut  away.  The  fragmental  products  are  always  contempo- 
raneous, and  when  interstratified  with  sediments  are  newer  than 
the  underlying,  older  than  the  overlying  stratum. 

II.     ROCKS   SOLIDIFIED    BELOW   THE   SURFACE    (PLUTONIC) 

We  now  come  to  a  series  of  rocks  which  no  one  has  ever  ob- 
served in  the  course  of  formation,  because  they  were  solidified  at 
greater  or  less  depths  beneath  the  ground.  When  such  masses 
are  exposed  to  view,  it  is  not  because  they  have  been  brought  to 
the  surface,  but  because  the  surface  has  been  eroded  down  to 
them.  Though  these  unstratified  masses  cannot  be  observed  in 
the  process  of  formation,  as  may  the  lavas  and  pyroclastic  rocks, 
yet  the  nature  of  the  rocks  themselves,  and  their  relations  to  the 
volcanic  and  stratified  rocks,  enable  us  to  explain  them  satisfac- 
torily. In  whatever  shape  they  occur,  these  masses  are  intrusive, 
and  have  forced  their  way  upward,  filling  fissures  and  cavities, 


278 


UNSTRATIFIED    ROCKS 


or  have  thrust  themselves  between  strata,  following  the  path  of 
least  resistance.  Intrusions  a.re  younger,  it  may  be  vastly  so,  than 
the  strata  which  they  penetrate  and  lie  over  or  beneath ;  their 
geological  date  may  be  determined  by  a  process  of  elimination, 
finding  the  newest  strata  which  they  have  traversed  and  the  oldest 
which  they  have  not  reached. 

Different  names  are  given  to  these  subterranean  masses,  in 
accordance  with  their  shape,  size,  and  relation  to  the  strata  with 
which  they  are  associated. 

Dikes.  — A  dike  is  a  vertical  or  steeply  inclined  wall  of  igneous 
rock  which  was  forced  up  into  a  fissure  when  molten  and  there 


FIG.  122.  —  Diagram  of  dike. 

consolidated.  Dikes  of  a  certain  kind  may  actually  be  seen  in 
the  making,  as  when  the  lava  column  of  a  volcano  bursts  its  way 
through  fissures  in  the  cone.  The  ordinary  dike  is  formed  in  fis- 
sures which  traverse  stratified  rocks,  or,  sometimes,  cuts  through 
older  and  already  consolidated  igneous  rocks.  In  thickness  dikes 
vary  from  a  foot  to  a  hundred  feet  or  more,  and  pursue  nearly 
straight  courses,  it  may  be  for  many  miles.  The  rock  of  a  dike  has 
usually  a  compact  texture,  having  cooled  more  slowly  than  the 
volcanic  masses,  though  the  edges,  chilled  by  contact  with  the  walls 
of  the  fissure,  may  be  glassy.  If  the  rock  displays  columnar  joint- 
ing, the  prisms  are  horizontal,  normal  to  the  cooling  surfaces. 


INTRUSIVE   MASSES  2/9 

The  commonest  rocks  in  dikes  are  basalt,  quartz  porphyry, 
andesite,  and  diabase.  * 

When  denudation  has  so  far  cut  away  the  surface  of  the  ground 
as  to  expose  the  dike,  the  form  which  the  latter  takes  will  depend 
upon  the  relative  destructibility  of  the  igneous  rock  and  the  enclos- 
ing strata.  If  the  latter  wear  away  more  rapidly,  the  dike  will  be 
left  standing  above  the  surface  like  a  wall  (Fig.  122)  ;  but  if  the 
igneous  mass  be  disintegrated  more  rapidly  than  the  strata,  a  trench 
will  mark  the  line  of  the  dike. 


FlG.  123.  —  Dike  of  basalt  cutting  strata:  bad  lands  of  eastern  Oregon. 

Dikes  are  common  and  conspicuous  objects  in  the  Connecticut 
valley  and  in  the  sandstone  belt  which  runs,  with  interruptions, 
from  the  Hudson  River  to  North  Carolina. 

Veins  are  smaller  and  more  irregular,  frequently  branching 
fissures  which  have  been  filled  with  an  igneous  magma ;  they  may 
be  only  a  few  inches  in  thickness,  and  may  often  be  traced  to  the 
mass  which  gave  them  off.  The  nature  of  the  rock  in  a  vein  may 
be  much  modified  by  material  derived  from  the  walls.  This  vein 
rock  is  often  so  coarsely  crystalline,  that  it  has  been  suggested 
that  it  could  not  have  solidified  from  fusion,  but  was  deposited 
from  a  solution  in  superheated  waters. 


280 


UNSTRATIFIED    ROCKS 


Intrusive  Sheets  or  Sills.  —  These  are  horizontal  or  moderately 
inclined  masses  of  igneous  rock,  which  have  small  thickness  as 
compared  with  their  lateral  extent.  Sheets  conform  to  the  bed- 
ding planes  of  the  strata,  often  running  long  distances  between  the 


FIG.  124.  — Sheet  of  jointed  diabase.     Orange,  NJ.     (U.  S.  G.  S.) 

same  two  beds  ;  but  if  they  can  be  traced  far  enough,  they  may 
generally  be  found  cutting  across  the  strata  at  one  point  or 
another.  In  thickness  they  vary  from  a  few  feet  to  several  hun- 
dreds of  feet.  The  Palisades  of  the  Hudson  are  formed  by  a 
sheet  of  unusual  thickness ;  its  outcrop  is  70  miles  long  from 


INTRUSIVE   SHEETS  28 1 

north  to  south,  and  its  thickness  varies  from  300  to  850  feet ;  the 
dike  which  supplied  this  immense  mass  is  exposed  in  a  few  places 
along  its  western  edge. 

Intrusive  sheets  are  most  commonly  formed  in  horizontal  strata, 
which  offer  less  resistance  to  horizontal  expansion  than  do  the 
folded  beds ;  they  are  also  very  generally  of  the  most  fusible 
family,  the  basaltic,  because  such  magmas  retain  their  fluidity  and 


FlG.  125.- — Palisades  of  the  Hudson,  New  Jersey.     (Photograph  by  Rau.) 

flow  for  longer  distances  than  do  the  highly  siliceous  rocks.  It  is 
probable  that  intrusive  sheets  can  be  formed  at  only  moderate 
depths,  because  the  overlying  strata  must  be  lifted  to  an  amount 
equal  to  the  thickness  of  the  sheet.  At  great  depths  the  weight 
to  be  lifted  is  so  enormous,  that  the  easiest  path  of  escape  must 
be  by  breaking  through  and  across  the  strata.  If  the  beds  are 
subjected  to  compression  after  the  intrusion  of  the  igneous 
masses,  the  latter  will  be  flexed  or  faulted  like  the  stratified 
rocks. 


282 


UNSTRATIFIED   ROCKS 


In  a  limited  exposure  it  is  often  difficult  to  distinguish  at  once 
between  an  intrusive  and  a  contemporaneous  sheet,  but  there  are 
certain  characteristic  marks  which  enable  the  observer  to  decide. 
The  presence  of  scoriae  shows  that  the  sheet  is  contemporaneous. 
If,  on  the  other  hand,  the  overlying  stratum  be  baked  and  altered 
by  the  heat,  or  if  the  sheet  cuts  across  the  bedding  planes  at  any 


FlG.  126.  —  Contact  of  intrusive  sheet  of  diabase  with  shales. 
Wiehawken,  NJ.     (U.  S.  G.  S.) 


Base  of  Palisades, 


point,  or  if  it  can  be  traced  to  a  dike  which  rises  above  it,  or  if  it 
gives  off  tongues  or  veins,  or  if  pieces  of  the  overlying  stratum 
be  torn  off  and  included  in  the  sheet,  it  must  be  intrusive.  The 
nature  of  the  contact  between  the  sheet  and  the  stratum  above  it 
is  also  significant ;  if  the  former  be  contemporaneous,  the  cracks 
and  fissures  of  its  upper  surface  will  be  filled  with  the  sediment- 
ary material.  Finally,  the  texture  of  the  igneous  mass  gives  valu- 
able evidence ;  in  the  intrusive  sheet  the  texture  is  compact 


LACCOLITHS 


283 


(without  glassy  ground  mass)  or  even  quite  coarsely  crystalline, 
while  the  contemporaneous  sheet  will  display  the  glassy  or  por- 
phyritic  texture  of  surface  flows. 

Laccoliths.  —  A  laccolith  (or  laccolite)  is  a  large,  lenticular 
mass  of  igneous  rock,  filling  a  chamber  which  it  has  made  for 
itself  by  lifting  the  overlying  strata  into  a  dome-like  shape.  The 
rock  of  which  laccoliths  are  made  is  nearly  always  of  the  highly 
siliceous  and  less  fusible  kinds,  so  that  it  can  more  easily  lift  the 
strata  than  force  its  way  between  them.  Intrusive  sheets  are,  it 
is  crue,  often  given  off  from  a  laccolith,  but  these  are  of  quite  sub- 


FIG.  127.  —  Diagram  of  uneroded  laccolith.     (Modified  from  Gilbert.) 

ordinate  importance,  while  dikes  and  irregular  protrusions  extend 
into  the  fissures  of  the  surrounding  and  overlying  strata.  Sub- 
sequent erosion  may  remove  the  dome  of  strata  and  cut  deeply 
into  the  igneous  mass  beneath,  leaving  rugged  mountains,  the 
height  of  which  depends  upon  the  amount  of  original  uplift  and 
the  subsequent  denudation.  Laccoliths  in  various  stages  of  de- 
nudation occur  in  different  parts  of  the  West.  Fig.  128  shows 
Little  Sun  Dance  Hill  in  South  Dakota,  a  small  dome  from  which 
the  overarching  strata  have  not  been  removed  and  the  igneous 
core  has  nowhere  been  exposed,  yet  there  can  be  little  doubt  of 
its  presence.  In  the  same  region  is  Mato  Tepee  (also  called  the 


284  UNSTRATIFIED   ROCKS 

Devil's  Tower),  a  magnificent  shaft  of  columnar  phonolite,  which 
rises  700  feet  above  a  platform  of  horizontal  strata.  This  tower 
is  the  remnant  of  a  laccolith  from  which  the  covering  strata,  and 
probably  much  of  the  igneous  core,  have  been  eroded  away.  In 
southern  Utah  the  Henry  Mountains  are  a  group  of  laccoliths 
from  which  several  thousand  feet  'of  overlying  strata  have  been 
removed  and  the  cores  deeply  dissected.  In  the  Elk  Mountains 
of  Colorado  are  some  enormous  laccolithic  masses. 


FIG.  128.  — Little  Sun  Dance  Hill,  South  Dakota.     (U.  S.  G.  S.) 

Bosses  are  rounded  or  irregular  masses  of  intrusive  rock,  which 
may  be  only  a  few  feet  or  several  miles  in  diameter.  Their  ex- 
posure on  the  surface  is  due  to  the  removal  of  overlying  strata, 
and  their  prominence  as  hills  is  caused  by  their  greater  resistance 
to  denudation  than  that  of  the  enclosing  stratified  rocks.  Some 
bosses  are  believed  to  be  the  subterranean  reservoirs  which  once 
supplied  volcanoes ;  but  this  can  rarely  be  proved,  because  when 
the  boss  is  exposed  by  denudation,  the  volcanic  neck  has  been 
swept  away.  However  this  may  be,  many  bosses  probably  never 
communicated  with  the  surface  by  any  vent.  Veins,  dikes,  sheets, 
and  various  irregular  protrusions  are  frequently  given  off  from 
bosses.  Bosses  are  made  up  of  the  granitoid,  compact  or  por- 
phyritic  members  of  various  rock  groups  :  granite,  diorite,  basalt, 
and  gabbro  are  especially  common,  and  the  coarseness  of  the 


MATO  TEPEE 


285 


286  UNSTRATIFIED    ROCKS 

component  crystals  usually  increases  from  the  circumference  to 
the  centre  of  the  mass. 

Bathyliths  are  huge  masses  of.  igneous  rock,  which  may  be 
scores  or  hundreds  of  miles  in  extent,  and  are  of  entirely  irregular 
shape.  Like  bosses,  they  are  exposed  to  view  only  when  de- 
nudation has  cut  the  surface  down  to  the  level  at  which  they 
were  formed.  It  is  difficult  to  understand  how  such  vast  quanti- 
ties of  material  could  have  been  forced  upward,  except  by  melt- 
ing their  way,  at  least  partly,  through  the  overlying  rocks. 

From  this  brief  description  it  will  be  apparent  that  the  various 
forms  of  igneous  rock  which  present  themselves  to  our  study  are 
the  outcome  of  the  interaction  of  several  factors,  such  as  the 
ascensive  pressure,  the  resistance  to  be  overcome,  and  the  fluidity 
or  stiffness  of  the  molten  magma.  Such  masses  have  played  a 
very  important  role  in  the  modification  of  the  earth's  surface, 
both  by  the  displacement  of  previously  existing  rocks  and  by  the 
addition  of  new  and  different  material. 


CHAPTER  XVI 

METAMORPHISM  AND  METAMORPHIC  ROCKS 

BY  the  term  metamorphism  is  meant  the  profound  transforma- 
tion of  a  rock  from  its  original  condition  by  means  other  than  those 
of  disintegration.  The  incipient  changes  of  the  latter  class  may 
very  greatly  modify  a  rock  and  its  constituent  minerals,  but  such 
changes  are  distinguished  from  metamorphism  under  the  term 
alteration.  Metamorphism  usually  implies  an  increase  in  hardness 
and  in  the  degree  of  crystallization,  and  very  frequently  also  the 
generation  of  an  entirely  new  set  of  minerals,  which  take  on  a 
characteristic  arrangement.  The  degree  of  metamorphism  varies 
according  to  circumstances,  and  from  the  mere  consolidation  of 
loose  sediments  to  the  most  radical  reconstruction  of  the  rock, 
there  is  every  possible  transition.  Fossils  may  be  found  in  those 
metamorphic  rocks  of  sedimentary  origin,  which  have  not  been 
completely  changed.  The  more  thorough  the  reconstruction  of 
the  rock,  the  more  obscure  do  the  fossils  become,  and  in  advanced 
stages  nearly  all  trace  of  them  is  obliterated. 

For  many  years  it  was  supposed  that  the  metamorphic  rocks 
were  one  and  all  transformed  sediments,  but  later  investigations 
have  shown  that  many  of  them  were  originally  igneous.  Indeed, 
it  is  often  quite  impossible  to  decide  whether  a  given  metamorphic 
rock  has  been  derived  from  a  sedimentary  or  an  igneous  original. 
This  is  not  surprising,  for  the  ultimate  chemical  (not  the  minera- 
logical)  composition  of  a  basalt,  a  volcanic  tuff,  or  a  clay  shale, 
may  be  the  same,  and  the  metamorphic  processes  may  produce 
an  identical  rock  from  any  one  of  these  three  as  a  starting-point. 
Much  yet  remains  to  be  learned  regarding  the  modes,  causes,  and 
results  of  metamorphism  and  some  of  the  most  far-reaching  prob- 
lems of  geology  are  bound  up  with  these  questions. 

287 


288  METAMORPHISM 

Metamorphism  is  of  two  quite  distinct  kinds:  (i)  contactor 
local,  and  (2)  regional  metamorphism. 


I.    CONTACT   METAMORPHISM 

This  is  the  change  effected  in  surrounding  rocks  by  igneous 
intrusions,  dikes,  bosses,  etc.  The  rock  invaded  and  metamor- 
phosed may  be  either  sedimentary,  igneous,  or  already  metamor- 
phic,  and  the  effects  may  be  very  marked,  or  surprisingly  small ; 
indeed,  it  is  often  quite  impossible  to  say  why  the  changes  should 
be  so  insignificant.  Plutonic  rocks  are  more  effective  in  producing 
these  changes,  because  they  are,  presumably,  hotter  and  retain 
their  heat  longer.  Magmas  which  contain  an  abundance  of  the 
mineralizing  vapours  (see  p.  192)  produce  much  more  effect  than 
those  with  only  a  small  quantity  of  such  vapours.  For  this  reason 
acid  magmas  are  more  effective  than  basic.  Much,  too,  depends 
upon  the  nature  of  the  invaded  rock;  sediments  which  contain 
large  percentages  of  alumina  and  lime  are  much  more  readily  and 
profoundly  changed  than  those  which  are  made  up  almost  entirely 
of  silica.  The  distance  to  which  the  zone  of  change  extends  is 
wider  when  the  intrusive  mass  cuts  across  the  strata  than  when  it 
follows  the  bedding  planes,  so  that  a  dike  or  boss  is  more  effective 
than  a  sheet. 

We  may  now  consider  some  examples  of  contact  metamorphism, 
and,  for  this  purpose,  shall  select  only  the  changes  of  sedimentary 
rocks ;  for  those  of  the  other  classes  require  a  treatment  too  minute 
and  refined  for  an  elementary  work.  We  may  note,  in  passing, 
however,  that  some  of  the  veins  given  off  from  granite  bosses, 
which  have  invaded  other  igneous  rocks,  are  probably  of  a  meta- 
morphic  nature  and  due  to  the  penetration  of  vapours. 

In  a  series  of  strata  which  have  been  invaded  by  an  igneous 
mass,  we  find  a  gradual  change  from  the  unmodified  rock  which 
lies  beyond  the  reach  of  the  transforming  agencies,  to  that  at  the 
actual  contact  with  the  igneous  mass.  Along  this  line  of  contact 
the  strata  are  so  thoroughly  reconstructed  that  often  only  a  micro- 
scopical examination  will  distinguish  the  changed  sediment  from 


CEMENTATION  289 

the  igneous  rock.  A  siliceous  sandstone  or  conglomerate  de- 
velops no  new  minerals  in  the  change,  or  only  in  insignificant 
quantity  from  the  impurities  present.  The  bulk  of  the  material 
simply  crystallizes  and  forms  the  white  rock,  quartette.  Clay 
rocks  undergo  more  radical  change  and  are  usually  divisible  into 
distinct  zones ;  the  outermost  zone  is  unchanged  ;  in  the  inter- 
mediate one  the  shale  is  changed  to  a  dense  slate  spotted  with 
biotite,  magnetite,  or  other  dark  minerals.  The  spotted  slate 
passes  gradually  into  mica  schist,  a  rock  made  up  of  flakes  of 
mica,  with  some  quartz  and  felspar,  arranged  in  rudely  parallel 
planes.  At  the  contact  the  rock  is  converted  into  hornfels?  which 
is  a  very  dense  substance,  looking  like  trap,  and  filled  with  numer- 
ous silicated  minerals,  such  as  hornblende,  felspar,  and  many 
others  which  were  not  enumerated  in  the  chapter  on  the  rock- 
forming  minerals. 

Pure  limestone  is  crystallized  by  the  heat  into  white  marble, 
but  as  most  limestones  contain  impurities,  they  develop,  when 
metamorphosed,  a  large  variety  of  minerals,  such  as  biotite,  gar- 
net, amphiboles,  pyroxenes,  etc.  Beds  of  bituminous  coal  are 
baked  into  a  natural  coke,  as  in  Virginia  and  North  Carolina,  or 
changed  to  anthracite,  as  in  Colorado,  and  limonite  is  converted 
into  magnetite. 

In  contact  metamorphism,  the  mere  molecular  rearrangement 
and  chemical  recombination  of  materials  already  present  in  the 
rock  are  not  the  only  changes  which  occur.  Two  other  processes, 
cementation  and  injection,  frequently  produce  important  results. 
Cementation  is  the  deposition  of  mineral  matters  from  solution  in 
the  interstices  between  the  granules  of  the  rock.  Quartz,  calcite, 
iron  oxides,  felspars,  mica,  augite,  and  other  minerals  may  be 
thus  introduced,  and  sometimes  the  quantity  of  new  material 
brought  into  the  rock  is  very  large.  Injection  is  the  penetration 
of  a  rock  by  molten  substances  which  may  not  only  fill  up  all 
the  minute  crevices,  but  even  force  their  way  between  the  con- 
stituent granules.  The  distinction  between  cementation  and 

1  Also  called  hornstone,  but  as  this  term  is  used  for  flint,  it  is  best  to  retain  it  in 
the  latter  sense  only. 
U 


2QO  METAMORPHISM 

injection  is  not  a  very  sharply  marked  one,  because  superheated 
water  and  molten  magmas  appear  to  mix  in  all  proportions.  The 
difference  between  the  two  processes  seems  thus  to  be  largely  a 
question  of  the  quantity  of  water  present. 

Contact  metamorphism,  as  its  name  implies,  is  a  local  phenom- 
enon, but  a  widely  ramifying  and  complex  system  of  igneous 
intrusions  may  change  large  areas  of  sedimentary  rocks. 

II.   REGIONAL  METAMORPHISM 

This  term  applies  to  the  reconstruction  of  rocks  upon  a  great 
scale,  in  areas  covering,  it  may  be,  thousands  of  square  miles,  and 
evidently  other  processes  in  addition  to  those  of  contact  meta- 
morphism are  needed  to  explain  such  wide-spread  changes.  A 
very  general  characteristic  of  such  metamorphic  rocks  is  foliation, 
or  schistosity.  This  is  either  cleavage  or  fissility  (see  p.  260),  or 
both,  which  causes  the  rock  to  part  into  plates  with  rough  or 
undulating  surfaces,  due  to  the  presence  of  flakes  of  some  mineral 
arranged  in  rudely  parallel  planes.  Schistosity  is  connected  by 
every  transition  with  cleavage  or  fissility,  and  represents  an  ad- 
vanced degree  of  metamorphism,  as  the  latter  processes  are 
incipient  stages  of  the  same. 

The  first  step  in  metamorphism  consists  in  a  mere  hardening 
of  the  rock,  accompanied  with  the  loss  of  water  and  other  vola- 
tile substances.  In  the  second  stage  the  component  minerals 
are  crystallized,  but  new  compounds  are  sparingly  formed.  The 
shearing  or  crushing  to  which  the  mass  has  been  subjected  fre- 
quently change  minerals  into  paramorphic  forms,  i.e.  those  which 
have  the  same  chemical  composition,  but  different  crystalline 
form  and  physical  properties.  For  example,  aragonite  is  thus 
changed  to  calcite,  and  augite  to  hornblende.  This  stage  is  fre- 
quently accompanied  by  cleavage  or  schistosity.  In  the  more 
advanced  stages  the  rocks  are  foliated,  and  complete  chemical 
reorganization  may  take  place,  with  the  abundant  development 
of  new  minerals.  The  compression  and  consequent  shearing  and 
crushing  to  which  the  rocks  have  been  subjected  are  the  princi- 


CAUSES   OF   METAMORPHISM  29 1 

pal  agents  of  the  changes,  though  igneous  intrusions  frequently 
add  very  materially  through  the  extensive  development  of  contact 
metamorphism. 

The  igneous  rocks,  when  subjected  to  the  same  processes,  give 
rise  to  rocks  similar  to  those  made  from  the  metamorphism  of 
sediments.  The  compression,  shearing,  and  crushing  may  take 
place  while  the  molten  mass  is  still  pasty,  or  long  after  the  rock 
has  cooled.  Certain  rocks  have  been  formed  from  the  meta- 
morphism of  sediments  and  the  injection  of  igneous  material,  and 
are  thus  of  highly  complex  origin. 

III.  THE  CAUSES  OF  METAMORPHISM 

This  is  a  subject  which  bristles  with  difficulties,  and  of  which 
our  knowledge  is  yet  very  incomplete,  though,  in  a  general  way, 
the  agencies  of  the  transformation  are  intelligible. 

(1)  Heat  is  evidently  a  very  important  factor  of  the  change. 
This  is  made  plain  by  the  phenomena  of  contact  metamorphism 
and  by  the  numerous  successful  attempts  to  imitate  metamorphism 
experimentally.     On  the  other  hand,  it  is  not  believed  that  high 
temperatures  are  always  indispensable.     Change  in  which  heat  is 
the  principal  factor  is  called  thermal  metamorphism. 

(2)  Compression   is   the    principal   agency   in    regional   meta- 
morphism, and  to  it  are  due  the  structures  of  cleavage,  fissility, 
and  schistosity,  as  well  as  the  reconstruction  and  crystallization  of 
mineral  particles.     This  is  dynamic  metamorphism,  but  heat  is 
probably  ^  common  accessory  in  this  method  of  change  also. 
To  be  in  the  zone  of  flovvage,  rocks  must  be  so  deeply  buried 
that  they  are  invaded  by  the  earth's  internal  heat ;  and  unless  the 
movement  be  excessively  slow,  flowage  must  generate  frictional 
heat. 

(3)  Moisture  is  likewise   a  potent  cause  of  change.     Under 
pressure,  water  may  be  heated  to  very  high  temperatures,  when  it 
becomes  capable  of  attacking  and  dissolving  or  decomposing  the 
most  refractory  substances,  and  building  them  up  into  new  com- 
pounds.    Many  minerals,  such  as  orthoclase  and  quartz,  which 


2Q2  METAMORPHISM 

have  never  been  experimentally  made  by  dry  heat,  may  be 
readily  compounded  and  crystallized  with  the  aid  of  superheated 
water.  Furthermore,  the  presence  of  water  diminishes  the  tem- 
perature necessary  to  affect  metamorphic  changes ;  and  rocks 
which  require  a  heat  of  2500°  F.  to  melt  them  in  the  dry  state, 
will  in  the  presence  of  water  become  pasty  and  viscous  at 
750°  F.  In  contact  metamorphism  other  mineralizing  vapours 
and  gases  play  an  important  part. 

(4)  Pressure  is  a  necessity  for  any  extensive  metamorphism, 
whether  thermal  or  dynamic,  to  produce  the  necessary  plastic 
flow  or  shearing  of  the  rocks,  and  to  prevent  the  escape  of  the 
steam  and  gases.  Limestone  heated  in  an  open  vessel  becomes 
quicklime  (CaO),  because  the  CO2  is  driven  off  at  high  tempera- 
tures. Heated  under  pressure,  the  same  limestone  will  crystal- 
lize into  marble.  On  a  large  scale,  therefore,  metamorphism  can 
be  effected  only  at  considerable  depths ;  for  it  is  in  the  zone  of 
flowage  that  the  most  favourable  conditions  are  to  be  found. 

It  is  believed  by  certain  geologists  that  metamorphism  may 
proceed  so  far  as  to  completely  melt  a  sedimentary  rock  and 
thus  produce  a  magma  which  is  indistinguishable  from  a  typically 
igneous  one.  Some  have  even  maintained  that  the  lavas  now 
ejected  from  volcanoes  are  but  the  final  results  of  metamorphism. 
These  conceptions  may  possibly  approximate  the  truth,  but  the 
progress  of  investigation  is  at  present  leading  away  from  them. 
No  instance  is  yet  known  which  renders  it  necessary  to  assume 
that  a  given  igneous  rock  was  made  from  melted  sediments,  and 
the  cases  which  have  been  relied  upon  to  prove  the  hypothesis 
have,  for  the  most  part,  been  shown  not  to  require  such  an 
explanation.  On  the  other  hand,  certain  metamorphic  rocks 
do  form  a  common  meeting-place  for  the  igneous  and  sedi- 
mentary classes,  and,  as  we  have  seen,  it  is  frequently  impossible 
to  decide  from  which  class  a  given  metamorphic  rock  was  origi- 
nally derived. 

Owing  to  this  uncertainty  regarding  their  derivation,  the  meta- 
morphic rocks  of  igneous  origin  are,  to  some  extent,  included 
with  those  formed  from  sediments  in  the  schemes  of  classification. 


GREYWACKE  293 

IV.   METAMORPHIC  ROCKS 

A.    NON-FOLIATED    ROCKS 

These  represent  the  less  advanced  stages  of  metamorphism,  in 
which  the  forces  of  compression  may  have  produced  cleavage  or 
fissility,  but  not  foliation.  The  more  important  rocks  of  this 
class  are  of  sedimentary  origin,  and  it  will  be  unnecessary  for  us. 
to  consider  the  igneous  rocks  which  have  been  changed,  though 
not  to  the  extent  of  producing  foliation. 

Quartzite  is  derived  from  the  metamorphosis  of  sandstone,  and 
between  the  two  kinds  of  rock  are  found  such  complete  transitions, 
that  the  separation  of  them  seems  almost  arbitrary.  In  a  typical 
quartzite  the  rock  is  crystalline,  and  the  quartz  deposited  around  the 
sand-grains  is  in  crystalline  continuity  with  those  grains,  though 
the  microscope  still  reveals  the  original  fragmental  nature  of  the 
rock.  Quartzites  also  result  from  the  metamorphism  of  conglom- 
erates, and  the  pebbles  are  sometimes  much  flattened  by  compres- 
sion. If  the  sandstone  or  conglomerate  contained  impurities,  other 
minerals  beside  quartz  are  generated  ;  if  any  considerable  quantity 
of  clay  was  present,  mica  will  be  produced  and,  it  may  be,  in  such 
abundance  that  the  rock  passes  into  mica  schist  (see  below). 

Quartzites  are  formed  both  in  contact  and  regional  metamor- 
phism, but  the  change  is  principally  due  to  cementation,  large 
amounts  of  silica  (estimated  as  one-sixth  of  the  original  quantity 
present  in  the  sandstone)  being  brought  in  and  deposited  from 
solution.  Many  quartzites  do  not  appear  to  have  been  subjected  to 
great  compression,  though  others  are  cleaved  or  fissile  (Fig.  113). 

Greywacke  is  a  hard,  crystalline  rock,  of  banded  appearance 
and  characteristic  grey  colour.  The  sedimentary  original  is  either 
a  mudstone,  an  arkose,  or  a  mass  of  fragments  of  felspar,  mica, 
quartz,  and  other  igneous  minerals  which  have  been  mechanically 
abraded,  but  little  or  not  at  all  decomposed.  The  change  is  due 
largely  to  cementation,  the  sand-grains  being  enlarged  as  in  quart- 
zites, and  silica  is  deposited  in  the  interstices,  binding  the  whole 
mass  firmly  together.  The  other  minerals  undergo  a  great  variety 


294  METAMORPHIC   ROCKS 

of  changes,  according  to  the  amount  of  compression  and  mashing 
which  the  rock  has  undergone.  Greywacke-slate  is  fine-grained 
and  parts  into  plates  parallel  with  the  stratification  planes. 

Slate  and  Phyllite.  —  Slate  is  a  fine-grained,  dense,  and  hard 
rock,  which,  when  metamorphosed  by  compression,  is  cleaved.  It 
results  from  the  transformation  of  clay  shales,  fine  arkose,  and 
sometimes  of  volcanic  tuffs.  Crushed  fragments  of  felspar  change 
•into  interlocking  crystals  of  quartz  and  felspar,  or  quartz  and  mica. 
The  mineral  particles,  both  original  and  newly  developed,  have 
a  parallel  arrangement  of  their  long  axes  and  cleavage  planes, 
which  determines  the  cleavage  of  the  rock.  In  colour,  slates  are 
usually  drab,  or  dull  dark  blue,  but  they  may  be  brick-red,  green, 
or  purple.  When  fine-grained  and  regularly  cleaved,  they  are  ex- 
tensively quarried  for  roofing  purposes.  Great  areas  of  them  occur 
in  Vermont,  eastern  Pennsylvania,  Virginia,  and  Georgia,  south  of 
Lake  Superior,  and  on  the  western  flank  of  the  Sierra  Nevada. 

Phyllite  is  slate  in  a  more  advanced  stage  of  metamorphosis,  in 
which  the  mica  spangles  are  more  abundant,  and  visible  to  the 
naked  eye,  giving  lustrous  surfaces  to  the  cleavage  planes.  Like 
micaceous  quartzite,  phyllite  may  often  be  traced  into  mica  schist. 

Marble  is  a  metamorphic  limestone,  in  which  the  fragments  and 
particles  of  organic  origin  have  been  converted  into  crystalline 
calcite.  Magnesian  limestones  yield  crystalline  dolomites,  which 
are  likewise  included  under  marble.  In  the  process  of  recon- 
struction, the  fossils  and  even  the  bedding  planes  of  the  original 
limestone  are  usually  entirely  obliterated.  The  grain  of  the  rock 
varies  much,  from  the  fine,  dense,  loaf-sugar-like  statuary  marble 
to  a  very  coarse  texture  of  large  crystals.  Pure  limestone  gives 
rise  to  a  white  marble,  but  the  presence  of  organic  matter  is  be- 
trayed by  veins  of  graphite,  which  may  indicate  the  lines  of  mash  • 
ing  and  flow,  along  which  the  rock  yielded  to  the  compressing 
force.  Iron  and  organic  matters  present  in  the  limestone  produce 
a  great  variety  of  coloured  and  variegated  marbles,  some  of  which 
are  of  extraordinary  beauty.  The  sand  and  clay  present  in  many 
limestones  will,  on  metamorphosis,  give  rise  to  a  variety  of  silicated 
minerals. 


FOLIATED    ROCKS  295 

Marble  is  an  exceptional  case  of  a  completely  crystalline  rock 
derived  from  sediments  by  dynamic  metaraorphism,  which  is  not 
foliated  or  schistose.  This  is  believed  to  be  due  to  the  capacity 
of  calcite  to  recrystallize  freely  after  it  has  been  subjected  to  com- 
pression and  mashing. 

The  economic  value  of  the  marbles  makes  them  largely  sought 
after ;  in  this  country  they  are  extensively  developed  along  the 
Appalachian  region,  from  Vermont  to  Georgia,  in  the  Rocky  Moun- 
tains, and  the  Sierra  Nevada. 

The  Ophicalcites  are  crystalline  magnesian  limestones  and  dolo- 
mites, with  varying  amounts  of  included  serpentine,  which  gives 
them  a  mottled  appearance.  They  are  not  thoroughly  understood, 
and  it  appears  that  they  may  be  formed  in  various  ways.  Some 
ophicalcites  are  almost  certainly  marbles,  in  which  inclusions  of 
olivine,  pyroxene,  or  hornblende  have  been  formed  and  afterwards 
altered  into  serpentine  (see  p.  22).  Others  would  appear  to  be 
broken  and  fissured  serpentines,  having  the  crevices  filled  up  with 
calcite  deposited  from  solution. 

Anthracite  is  usually  regarded  as  a  metamorphic  form  of  coal, 
and,  as  we  have  seen  in  a  preceding  paragraph  of  this  chapter,  it 
is  formed  from  bituminous  coal  by  contact  metamorphism.  On  a 
large  scale  it  occurs  chiefly  in  areas  of  folded  and  disturbed  rocks, 
though  not  invariably  so.  A  more  intense  metamorphism  of  car- 
bonaceous material  gives  rise  \a  graphite  (or  black  lead),  a  semi- 
crystalline  form  of  carbon,  which,  however,  is  a  mineral  rather 
than  a  rock. 

B.     FOLIATED    ROCKS 

The  foliated  or  schistose  rocks  are  those  which  are  divided  into 
rudely  parallel  planes,  with  rough  or  undulating  surfaces,  due  to 
the  flakes  and  spangles  of  some  mineral.  The  planes  of  foliation 
may  coincide  with  the  original  bedding  planes  or  they  may  inter- 
sect the  latter  at  any  angle,  just  as  do  the  planes  of  cleavage  and 
fissility.  The  foliated  rocks  represent  the  most  advanced  stage  of 
what  we  can  confidently  call  metamorphism,  and  may  be  derived 
from  either  sedimentary  or  igneous  originals;  it  is  not  always  pos- 
sible to  say  which. 


296 


METAMORPHIC   ROCKS 


Gneiss  is  a  term  of  wide  significance,  which  includes  a  number 
of  rocks  of  different  modes  of  origin  and  different  mineralogical 
composition.  It  is  "  a  laminated  metamorphic  rock  that  usually 
corresponds  in  mineralogy  to  some  one  of  the  plutonic  types " 
(Kemp).  The  varieties  of  gneiss  are  ordinarily  named  in  accord- 
ance with  the  most  conspicuous  dark  silicate  present,  as  biotite 
gneiss,  hornblende  gneiss,  etc. ;  but  this  system  of  nomenclature 
gives  an  imperfect  notion  of  the  character  of  the  rock.  A  better 
method,  recently  suggested  (C.  H.  Gordon),  is  to  name  the 


FIG.  130.  —  Plicated  gneiss.     (U.  S.  G.  S.) 

varieties  in  accordance  with  the  igneous  rocks  to  which  they 
correspond  in  mineralogical  composition ;  as  granitic  gneiss, 
syenitic  gneiss,  dioritic  gneiss,  etc.  The  commonest  variety  is 
granitic  gneiss,  with  mica  or  hornblende ;  the  orthoclase  and 
quartz  are  mingled  together,  with  conspicuous  laminae  and  folia 
of  the  dark  mineral. 

Most  gneisses  were  generated  by  the  dynamic  metamorphism  of 
granite,  either  before  its  consolidation  or  after  it  had  cooled  and 
hardened.  Some  authorities  deny  that  gneiss  has  ever  been  formed 
from  sedimentary  rocks,  but  there  is  good  reason  to  believe  that 


HORNBLENDE   SCHIST 

it  sometimes  has  such  an  origin,  and  in  certain  instances  the  crushed 
pebbles  of  the  parent  conglomerate  are  still  distinctly  visible.  Still 
another  series  of  these  rocks  are  of  complex  origin,  granitic  mag- 
mas being  injected  along  the  foliation  planes  and  into  all  the 
crevices  of  metamorphosed  sediments. 

Gneisses  are  widely  spread  in  ancient  formations,  especially  in 
the  most  ancient  of  all,  and  they  cover  vast  areas  in  the  northern 
part  of  North  America. 

The  Crystalline  Schists  are  more  finely  foliated  than  gneiss,  into 
which  they  often  grade  imperceptibly,  having  very  similar  miner- 
alogical  composition.  They  have  very  diverse  modes  of  origin 
arising  from  both  sedimentary  and  igneous  rocks.  Slates,  impure 
sandstones  and  limestones,  as  well  as  felsites,  andesites,  diabases, 
tuffs,  etc.,  may  all  give  rise  to  crystalline  schists  by  thermal  or 
dynamic  metamorphism.  The  varieties  are  named  from  their 
most  important  ferro-magnesian  mineral. 

Quartz  Schist  is  a  foliated  quartzite  in  which  cleavage  or  fissility 
has  developed  into  schistosity.  The  mashing  and  cementation  of 
the  original  sandstone  may  take  place  at  the  same  time,  or  the 
quartzite  may  be  produced  by  the  latter  process  and  subsequently 
converted  into  schist  by  compression. 

Mica  Schist  is  principally  composed  of  quartz,  muscovite,  and 
biotite,  with  more  or  less  felspar.  By  an  increase  in  the  quantity 
of  felspar  present,  and  a  coarser  foliation,  they  grade  into  gneiss, 
and  by  an  increase  of  quartz  they  may  pass  into  quartzite  and  thence 
to  sandstone.  Through  the  phyllites  mica  schists  are  connected 
with  the  slates,  and  in  another  direction,  by  increase  of  lime  they 
pass  into  argillaceous  limestones.  Mica  schists  are  very  largely 
exposed  in  New  England  and  southward  along  the  eastern  flank 
of  the  Appalachian  Mountain  system. 

Hornblende  Schist  is  a  foliated  rock,  consisting  of  hornblende 
with  a  varying  proportion  of  felspar  and  less  quartz.  The  horn- 
blende schists  are,  for  the  most  part,  derived  from  the  dynamic 
metamorphism  of  various  basic  igneous  rocks,  the  augite  being 
readily  converted  into  hornblende  by  crushing,  but  in  rare  instances 
they  are  believed  to  have  had  a  sedimentary  origin.  The  horn- 


298  METAMORPHIC   ROCKS 

blende  schists  occur  as  belts  or  bosses  in  metamorphic  areas  and 
are  largely  developed  around  Lake  Superior. 

The  schists  already  described  are  much  the  most  abundant 
varieties  of  the  group,  but  there  are  numbers  of  others.  Thus,  we 
have  talc  and  chlorite  schists,  both  of  which  are  due  to  alteration, 
principally  of  hornblende  schist,  and  graphite  schist,  which  has 
quantities  of  that  carbonaceous  mineral  along  its  foliation  planes. 

Summary.  — Structural  geology  brings  vividly  before  us  the 
innumerable  changes  through  which  the  earth's  surface  has  passed 
and  which  are  recorded  in  the  rocks.  The  sedimentary  rocks, 
originally  laid  down  under  water  in  approximately  horizontal  posi- 
tions, have  been  upheaved  into  land  surfaces,  either  without  losing 
that  horizontality,  or  being  tilted,  folded,  compressed,  or  even 
violently  overturned.  Or,  they  may  be  fractured  and  dislocated  in 
great  faults  and  thrusts.  These  movements  we  have  found  to  be 
due  to  enormous  lateral  compression  set  up  within  the  crust  of 
the  earth,  a  compression  probably  generated  by  the  shrinkage  of 
the  cooling  globe.  Whether  folding  or  faulting  shall  result  from  a 
given  compression  depends  upon  the  rigidity  of  the  strata,  upon 
the  load  which  overlies  them,  and  the  sudden  or  gradual  way  in 
which  compression  is  applied.  The  results  of  compression  on  a 
large  scale  are  accompanied  by  certain  minor  changes  not  less 
characteristic.  Compressed  rocks  are  cleaved,  fissile,  or  schistose, 
according  to  the  intensity  of  the  action,  and  whether  the  rocks 
affected  are  in  the  zone  of  flowage  or  of  fracture.  These  changes 
may  go  so  far  as  to  completely  reconstruct  the  minerals  of  the 
rocks,  destroying  the  old,  generating  new,  and  obliterating  the 
original  character  of  the  strata.  Thus,  displacements,  dislocations, 
cleavage,  fissility,  and  dynamic  metamorphism  are  but  the  varying 
results  of  lateral  compression,  acting  under  different  conditions. 

Another  class  of  rocks — the  igneous,  massive,  or  unstratified  — 
we  found  to  have  penetrated  and  overflowed  the  strata,  and  to  have 
consolidated  in  the  fissures  and  cavities  which  they  have  made  for 
themselves,  or  to  have  been  poured  out  freely  on  the  surface. 
According  to  the  circumstances  under  which  these  masses  have 
cooled,  the  resulting  rock  is  of  glassy,  porphyritic,  finely  or  coarsely 


SUMMARY  299 

crystalline  texture.  When  solidified  as  sheets  or  dikes,  the  igneous 
rocks  may  be  folded,  faulted,  cleaved,  or  metamorphosed  like  the 
strata,  and  when  a  region  has  been  long  and  repeatedly  subjected 
to  compression,  its  structure  may  become  excessively  complex, 
and  the  metamorphosis  of  its  rocks  so  complete  that  not  even  the 
most  careful  examination  will  suffice  to  distinguish  those  rocks 
which  were  originally  sedimentary  from  those  which  were  igneous. 
Our  study  has  taught  us  that  many  of  these  processes  go  on 
deep  within  the  earth's  crust,  and  hence  cannot  be  directly  ob- 
served, but  must  be  inferred  from  their  results.  Very  encouraging 
progress  has  already  been  made  in  this  work,  but  much  remains  to 
be  done  before  our  knowledge  of  structure  and  its  full  meaning 
shall  be  even  approximately  complete. 


PART    III 


PHYSIOGRAPHICAL   GEOLOGY 
CHAPTER    XVII 

LAND   SCULPTURE 

PHYSIOGRAPHICAL  geology  is  the  study  of  the  topographical  feat- 
ures of  the  earth,  and  of  the  means  by  which,  and  the  manner  in 
which,  they  have  been  produced. 

This  subject  is  primarily  a  department  of  physical  geography, 
but  is  of  value  to  the  geologist  for  the  light  which  it  throws  upon 
the  historical  development  of  the  land  surfaces,  and  upon  features 
of  the  past  which  are  not  recorded  in  the  processes  of  sedimenta- 
tion. The  geographer  endeavours  to  explain  the  topographical 
forms  of  the  land,  and,  in  order  to  do  this,  he  must  show  how 
those  forms  have  originated.  The  geologist,  on  the  other  hand, 
makes  use  of  the  topography  to  determine  what  changes  have 
passed  over  the  land,  and  in  what  order  those  changes  have 
occurred.  The  old  method  of  reading  geological  history  con- 
cerned itself  merely  with  the  sedimentary  accumulations  and 
igneous  intrusions.  This  method  has  the  defect  of  leaving  us  with- 
out information  regarding  the  changes  of  land  surfaces  (except 
where  transgressions  of  the  sea  are  recorded  in  unconformities) 
and  the  details  of  mountain- making.  The  physiographical  method 
supplements  this  by  adding,  in  part,  the  required  information  con- 
cerning the  land  surfaces.  Each  method  is  improved  and  strength- 
ened when  we  use  both  of  them  together,  and  when  we  are  able 

300 


TOPOGRAPHY  30 1 

to  correlate  the  accumulations  of  sediments  with  the  denuding 
processes  which  furnished  the  material. 

The  topography  of  any  land  area  may  be  considered  as  the 
outcome  of  a  struggle  between  two  opposing  sets  of  agencies  : 
( i )  those  which  tend  to  upheave  the  region  and  thus  increase  its 
elevation ;  (2)  those  which  tend  to  cut  down  the  land  to  the  level 
of  the  sea.  The  latter  comprise  the  agencies  of  denudation,  or 
degradation,  while  the  former  are  the  diastrophic  agencies,  or  simply 
called  diastrophism.  Two  kinds  or  manifestations  of  diastrophism 
may  be  distinguished:  (a)  Epeirogenic  (from  the  Greek  epeiros,  con- 
tinent), the  broad  uplifts  or  depressions  of  areas,  not  necessarily 
accompanied  by  folding  or  tilting  of  the  strata;  (b)  Orogenic 
(from  the  Greek  oros,  mountain),  the  upheaval  of  relatively 
narrow  belts  of  land,  caused  by  the  lateral  compression  of  the 
strata.  It  is  not  yet  known  whether  these  two  processes  differ 
in  principle,  or  whether  they  are  merely  different  manifestations 
of  the  same  agencies.  Sometimes,  indeed,  the  degrading  and  dia- 
strophic agencies  cooperate,  as  when  the  land  is  depressed  instead 
of  upheaved,  but  this  is  not  the  more  common  condition. 

The  details  of  topography  are,  in  large  degree,  controlled  by 
still  a  third  class  of  factors,  which,  however,  are  passive  rather 
than  active ;  namely,  the  character,  arrangement,  and  attitude  of 
the  rock  masses.  A  partially  degraded  region  in  which  the  rocks 
are  homogeneous  will  have  a  very  different  kind  of  relief  from  one 
in  which  the  rocks  are  heterogeneous  and  differ  materially  in  their 
powers  of  resistance  to  the  denuding  agents.  A  region  of  hori- 
zontal strata  will  give  rise  to  very  different  topographical  forms 
from  those  which  are  developed  in  areas  of  folded  or  tilted  strata. 
We  must  further  distinguish  between  regions  whose  topography  is, 
in  the  main,  due  to  constructive  processes  from  those  in  which 
denudation  has  prevailed.  Examples  of  such  constructive  forms 
are  volcanic  mountains,  and  plains  or  plateaus  formed  by  widely 
extended  lava  flows,  plains  newly  deserted  by  the  sea  and  due  to 
sedimentation,  alluvial  plains  of  rivers,  and  the  mounds,  ridges, 
or  sheets  of  drift  spread  out  by  the  action  of  glaciers  and  of  the 
waters  derived  from  their  melting. 


3<D2  LAND    SCULPTURE 

The  topography  of  any  region  is,  as  we  have  seen,  the  resultant 
of  the  very  complex  interaction  of  many  different  kinds  of  factors, 
and  is  subject  to  continual  change  according  to  definite  laws. 
Let  us  suppose,  in  the  first  instance,  a  region  newly  upheaved 
from  beneath  the  sea  into  dry  land'.  The  topography  of  such  an 
area  will  be  constructional,  due  entirely  to  the  processes  of  dias- 
trophism  and  accumulation,  and  characterized  by  the  absence  of 
a  highly  developed  system  of  drainage  by  streams.  The  coastal 
plain  of  the  middle  and  southern  Atlantic  States  is  an  example  of 
such  topography  but  slightly  modified. 

Next,  the  processes  of  denudation  begin  their  work  upon  the 
region.  The  sea  attacks  the  coast-line  by  cutting  it  back  in  one 
place  and  building  out  in  another,  until  a  condition  of  equilibrium 
is  attained.  Rivers  are  established,  adjusting  themselves  to  the 
structure  of  the  underlying  rocks,  and  cutting  deep,  trench-like 
valleys,  while  the  atmospheric  agencies  widen  out  the  valleys, 
slowly  wearing  down  and  washing  away  the  sides  and  tops  of  the 
hills.  This  is  the  stage  in  which  we  find  the  greatest  degree  and 
variety  of  relief,  and  it  may  be  called  the  stage  of  maturity,  as 
contrasted  with  the  first,  which  is  a  stage  of  youth.  The  continu- 
ance of  the  degrading  operations  will,  if  uninterrupted,  eventually 
wear  down  the  region  to  a  nearly  plane  surface,  through  which 
sluggish  streams  meander,  the  featureless  condition  of  old  age. 
When  the  process  is  complete,  the  country  is  said  to  be  base- 
levelled. 

The  term  age  as  applied  to  topographical  features  does  not 
mean  the  length  of  time  required  for  their  formation,  but  merely 
the  stage  of  development  which  they  have  attained.  The  length 
of  time  required  to  reach  a  given  stage  of  such  development  will 
vary  greatly  in  different  regions,  in  accordance  with  climatic  con- 
ditions, the  resistance  of  the  rocks,  their  altitude  above  sea-level, 
and  similar  factors.  An  area  of  resistant  rocks  in  an  arid  climate 
will  be  hardly  at  all  affected  in  the  time  that  a  mass  of  soft  rocks 
exposed  to  a  heavy  rainfall  will  be  cut  down  to  base-level. 

It  seldom,  if  ever,  happens  that  the  topographical  development ' 
of  a  region  proceeds  uninterruptedly  through  the  stages  of  youth, 


DENUDING   AGENTS  303 

maturity,  and  old  age.  Oscillations  of  level  introduce  new  condi- 
tions and  cause  the  work  of  denudation  to  start  afresh  with  re- 
newed energy,  or,  if  the  movement  be  one  of  depression,  it  will 
check  the  work  already  in  progress.  The  cycles  of  development 
are  thus  partial  rather  than  complete,  and  a  given  region  may  dis- 
play topographical  forms  dating  from  very  different  and  widely 
separated  cycles.  The  more  resistant  rocks  retain  the  features 
acquired  in  an  earlier  cycle,  while  the  weaker  and  more  destructible 
rocks  have  already  taken  on  the  forms  due  to  a  later  cycle.  A 
landscape  thus  often  includes  features  of  different  geological  dates, 
and  it  is  in  the  identification  of  these  that  the  value  of  the  physio- 
graphical  method  to  historical  geology  consists. 

In  the  production  of  new  topographical  forms,  old  ones  are 
more  or  less  completely  destroyed,  and  thus,  the  farther  back  in 
time  we  go,  the  fewer  subdivisions  are  recognizable,  and  only  the 
outlines  of  the  great  cycles  can  be  followed.  Very  ancient  features 
would  be  quite  obliterated  in  the  successive  cycles  of  develop- 
ment, were  they  not  sometimes  buried  under  the  sediments  of  an 
encroaching  sea.  A  subsequent  reelevation  of  the  area  into  land, 
and  a  stripping  away  of  the  covering  of  newer  sediments  by  the 
agencies  of  denudation,  will  again  bring  to  light  the  ancient  land 
surface  which  had  been  buried  for  ages. 

In  Part  I  we  have  already  studied  the  agencies  of  denudation, 
but  there  we  concerned  ourselves  principally  with  the  modes  of 
operation  of  those  agencies,  and  their  efficiency  in  destroying  old 
rocks  and  in  furnishing  material  for  the  construction  of  new. 
We  have  now  to  consider  these  agencies  from  a  somewhat  differ- 
ent point  of  view ;  to  determine  their  relative  shares  in  the  work 
of  cutting  the  land  down  to  base-level,  and  the  characteristic 
forms  of  land  sculpture  which  they  produce  at  the  various  stages 
of  their  work.  There  are  some  differences  of  opinion  among 
geologists  regarding  the  relative  efficiency  of  the  various  denuding 
agents.  English  authorities,  for  example,  attribute  more  impor- 
tance to  the  work  of  the  sea  than  do  the  French  or  American,  who 
regard  the  sea  as  an  agency  altogether  subordinate  to  those  of  the 
atmosphere  and  running  waters.  Thus,  de  Lapparent  calculates 


304  LAND    SCULPTURE 

that  the  amount  of  material  annually  removed  from  the  land  by 
the  sea  is  only  about  one  cubic  kilometre,  or  less  than  one- 
fifteenth  the  quantity  carried  away  by  the  subaerial  agencies. 

The  Sea. — The  work  of  the  sea  is  confined  to  the  coast-line, 
which  it  cuts  back  by  the  impact  of  its  waves  and  currents. 
Speaking  broadly,  the  waves  do  but  little  effective  work  below 
the  limits  of  low  tide,  and  advance  by  undermining  and  cutting 
down  the  cliffs  which  form  the  coast.  The  result  of  the  work 
is  to  form  a  platform  covered  by  shallow  water,  which  is  called 
a  plain  of  marine  denudation.  As  observed  in  actual  cases,  these 
platforms  are  narrow  ;  for  so  long  as  the  sea-level  remains  constant 
with  reference  to  the  land,  there  is  a  limit  to  the  effective  assault 
of  the  waves  upon  the  shore.  The  water  covering  the  platform 
is  very  shallow  and  only  in  exceptional  cases  do  the  waves  have 
sufficient  power  to  overcome  the  friction  of  a  wide  platform. 
The  material  removed  from  the  land,  especially  the  coarser  and 
heavier  parts  of  it,  are  piled  up  at  the  seaward  foot  of  the  plat- 
form and  help  to  extend  it  in  that  direction. 

An  example  of  a  plain  of  marine  denudation  is  found  on  the 
north  coast  of  Spain,  where  there  is  a  broad  platform  between  the 
mountains  and  the  sea,  almost  perfectly  flat.  This  plain  has  been 
uplifted  above  the  sea-level  and  has  been  little  dissected  by  the 
subaerial  agents.  Narrower  platforms,  still  in  process  of  exten- 
sion, may  be  observed  on  most  rocky  and  precipitous  coasts,  as 
those  of  Scotland,  Ireland,  and  France.  Along  a  slowly  sinking 
coast  the  platforms  may  be  cut  back  much  farther,  for  the  deep- 
ening water  prevents  the  loss  of  wave  power  by  the  friction  on  a 
shoal  bottom.  If,  on  the  other  hand,  the  coast  rises  at  intervals, 
a  series  of  terrace-like  platforms  will  be  cut. 

As  we  shall  see  in  the  following  section,  plains  may  be  produced 
by  the  work  of  the  subaerial  agencies,  and  it  is  often  important  to 
distinguish  between  the  plains  of  submarine  and  those  of  subaerial 
"origin.  This  distinction  cannot  always  be  made  with  certainty, 
but  not  unfrequently  the  plain  shows  unmistakable  signs  of  the 
manner  in  which  it  was  made.  In  the  plain  of  marine  denudation 
the  sediments  formed  from  the  waste  of  the  land  will  be  deposited 


THE   SEA  305 

upon  the  seaward  portion  of  the  platform,  or  upon  a  lower  level 
of  previous  formation.  Further,  this  sediment  will  show  by  its 
character  that  it  actually  was  derived  from  the  material  cut  away 
by  smoothing  the  plain,  and  the  whole  of  it,  even  its  bottom 
layers,  will  be  of  marine  origin.  In  such  a  plain  the  advancing 
sea  must  have  obliterated  the  stream  valleys  which  had  been 
excavated  when  the  region  was  land.  This  obliteration  will  be 
performed  partly  by  shaving  down  the  divides,  or  watersheds, 
between  the  streams  and  partly  by  filling  up  the  valleys  with 
sediment. 

When  the  region  is  once  more  uplifted  above  the  level  of  the 
sea,  an  entirely  new  system  of  drainage  will  be  established  upon 
it,  determined  by  the  slopes  of  the  overlying  cover  of  newly  de- 
posited sediments,  and  having  no  reference  to  the  structure  and 
arrangement  of  the  underlying  older  rocks.  These  newly  estab- 
lished streams  may,  if  the  upheaval  of  the  country  gives  them 
sufficient  fall,  cut  down  through  the  newer  sediments.  Indeed, 
the  latter  may  eventually  be  swept  away  entirely  by  the  various 
subaerial  agencies,  but  the  stream  courses,  which  were  determined 
originally  by  the  slopes  of  that  newer  sediment,  will  show  little  or 
no  adjustment  to  the  structure  of  the  underlying  older  rocks. 

These  criteria  are  useful  in  identifying  those  plains  which  were 
smoothed  by  the  action  of  the  sea ;  but  when  the  processes  of  sub- 
aerial  denudation  have  completely  dissected  the  elevated  area,  all 
such  evidences  may  be  removed  and  the  origin  of  the  plain  may 
become  quite  indeterminable. 

A  coast-line  newly  formed  by  the  elevation  of  land  may  be  dis- 
tinguished from  one  which  has  stood  long  at  the  same  level  by  its 
unworked  character,  the  first  effect  of  elevation  being  to  produce 
an  even,  regular  shore-line.  This  is  because  the  combined  effects 
of  erosion  and  sedimentation  tend  to  make  the  sea-floor  flat  and 
smooth,  and  an  elevation  of  such  a  floor  to  a  given  altitude  must 
produce  an  even  and  regular  shore.  Nearly  the  whole  west  coast  of 
South  America  is  an  example  of  this.  On  the  other  hand,  a  coast 
which  has  long  stood  at  the  same  relative  level  will  show  plainly 
the  long-continued  action  of  the  sea  upon  it.  What  results  that 
x 


306  LAND    SCULPTURE 

action  will  have  will  depend  largely  upon  the  character  and 
arrangement  of  the  rocks  which  make  up  the  coast,  and  whether 
the  shore  rises  steeply  or  gently  out  of  the  sea.  The  first  effect 
of  the  wearing  action  of  the  sea  is  to  increase  the  irregularity  of 
the  coast-line,  by  taking  advantage  of  the  weaker  spots,  with 
headlands  and  rocky  points  formed  of  the  more  resistant  portions 
(see  Fig.  42).  Eventually,  however,  the  headlands  are  cut  back, 
leaving  a  submarine  platform  to  mark  their  former  extension, 
while  from  this  platform  may  rise  islets  of  erosion,  formed  by 
masses  of  rocks  which  have  better  resisted  the  assaults  of  the 
waves.  In  areas  of  deposition  bars,  shoals,  and  sand  spits  are 
thrown  up  by  the  winds  and  waves,  running  parallel  with  the  coast, 
and  the  open  beaches  take  on  a  crescentic  form.  Long-continued 
action  of  the  sea  at  a  constant  level  thus  tends  to  approximate  the 
even  and  regular  coast-lines  of  the  newly  upheaved  land.  However, 
an  inspection  of  the  structure  of  the  coast  will  reveal  the  difference. 

A  coast  which  has  been  depressed  within  a  comparatively 
recent  period  is  one  of  marked  irregularity  of  outline.  The  ad- 
vancing sea  fills  all  the  lower  valleys,  while  the  higher  ridges  stand 
out  as  headlands  and  promontories.  Isolated  hills  and  mountains 
are  separated  from  the  mainland  and  converted  into  islands.  The 
rivers  are  drowned,  their  lower  courses  converted  into  estuaries, 
into  which  side  streams  empty  separately,  that  before  had  joined 
the  main  stream.  A  very  irregular  coast-line,  penetrated  by  many 
estuaries,  inlets,  or  fjords,  and  fringed  with  numerous  islands,  indi- 
cates the  submergence  of  a  region  already  carved  into  strong  relief 
by  the  subaerial  agencies.  Such  a  coast  is  admirably  typified  by 
that  of  Maine,  where  the  topography  was  evidently  modelled  by 
subaerial  denudation,  while  a  comparatively  recent  depression  has 
brought  the  sea  over  it.  The  actual  form  taken  by  a  lowered 
coast  will  depend  upon  the  degree  and  character  of  the  relief 
which  had  been  attained  before  the  depression,  and  upon  the 
amount  of  the  subsidence. 

The  depression  of  a  low-lying  coast  of  small  relief  is  made  mani- 
fest chiefly  in  the  invasion  of  the  rivers  by  tidal  waters  and  their 
conversion  into  estuaries,  examples  of  which  are  New  York,  Dela- 


SUBAERIAL   AGENTS 


307 


ware,  and  Chesapeake  Bays.  These  estuaries  show  by  their  form 
that  they  are  river  valleys  invaded  by  the  sea,  and  the  channel  of 
the  Hudson  has  been  traced  by  soundings  to  the  edge  of  the  con- 
tinental platform,  one  hundred  miles  out  from  Sandy  Hook. 

The  denuding  action  of  the  sea  upon  a  depressed  and  irregular 
coast-line  is  to  reduce  its  irregularities,  wearing  down  the  islands, 


cutting  back  the 


promontories,  and  filling 


up  the  bays  and  more 


FlG.  131.  —  Christiania  Fjord,  Norway.     (Photograph  by  Libbey.) 

sheltered  spots,  and,  if  long  enough  continued,  will  result  in  the 
same  evenness  of  contour  as  is  produced  by  the  working  of  an 
upheaved  shore. 

The  Subaerial  Agents  are  those  which  operate  over  the  entire 
surface  of  the  land.  Their  tendency  is,  in  the  first  instance,  to 
carve  out  valleys  and  leave  relative  eminences  standing,  and  thus 
to  increase  the  irregularity,  or  relief t  of  the  land.  This,  however, 
is  merely  a  temporary  stage,  and  if  time  enough  be  granted,  these 
agencies  will  sweep  away  the  irregularities  and  plane  the  entire 
region  down  to  the  base-level  of  erosion. 


308  LAND    SCULPTURE 

Rivers  cut  down  and  deepen  their  channels  so  long  as  their 
beds  have  sufficient  slope  and  fall.  The  banks  also  are  under- 
mined, as  the  current  swings  from  side  to  side,  and  frequently  fall, 
thus  widening  the  channel.  The  sides  of  the  trench,  unless  re- 
moved by  other  agencies,  will  be  as  steep  as  the  nature  of  the 
rock  material  will  allow.  Unassisted  river  action  will,  therefore, 
cut  nearly  vertical  trenches,  which  are  continually  deepened,  until 
the  base-level  is  reached.  Examples  of  such  river-cut  trenches 
are  the  Au  Sable  Chasm  (see  Fig.  32)  and  the  inner  gorge  of  the 
Grand  Canon  of  the  Colorado  (see  Frontispiece). 

The  trench-like  valley,  with  nearly  vertical  sides,  is,  however, 
not  the  usual  form  of  river  valley.  The  atmospheric  agencies,  the 
undermining  and  sapping  of  springs,  landslips,  and  the  like,  are 
continually  wearing  away  the  sides  of  the  excavation,  the  waste 
thus  produced  being  readily  carried  away  by  the  stream.  As  the 
upper  part  of  each  hillside  and  cliff  is  that  which  has  been  long- 
est exposed  to  the  denuding  agencies,  the  valley  will  be  widened 
at  the  top  more  than  at  the  bottom,  and  will  gradually  be- 
come widely  open,  unless  the  alternation  of  hard  and  soft  strata 
be  such  as  to  favour  the  retention  of  the  cliff-like  form  by  under- 
mining. 

The  rapidity  with  which  the  deep  and  narrow  trench  is  widened 
into  the  broad,  gently  sloping  valley  will  depend  upon  two  sets  of 
conditions,  (i)  Upon  the  climate,  which  is  as  much  as  to  say, 
the  intensity  with  which  the  denuding  forces  operate.  Canons 
and  narrow  gorges  are  much  more  frequent  in  arid  regions  than 
in  those  of  abundant  rainfall.  (2)  Upon  the  resistant  power  of 
the  rocks.  If  the  valley  sides  are  composed  of  rocks  which  yield 
readily  to  weathering,  the  trench  will  be  speedily  broadened,  while 
if  the  rocks  offer  great  resistance  to  chemical  and  mechanical  dis- 
integration, the  gorge-like  form  will  be  retained  very  much  longer. 
This  is  illustrated  by  almost  any  considerable  stream,  such  as  the 
Delaware  or  the  Potomac.  In  certain  places  the  valley  is  widely 
open,  while  in  other  parts  of  the  course  are  deep  gorges,  as  at  the 
Delaware  Water  Gap  and  Harper's  Ferry.  The  gorges  occur  in 
the  places  where  the  stream  cuts  across  hard,  resistant  rocks,  and 


SUBAERIAL  AGENTS  309 

the  open  valleys  where  it  intersects  softer  and  more  destructible 
rocks. 

Rivers  also  produce  changes  in  topography  by  constructional 
processes,  as  in  their  flood  plains  and  terraces,  processes  which 
are  most  notable  in  the  lower  parts  of  the  course,  and  which  gain 
increased  efficiency  through  a  subsidence  of  the  region. 

Degradation  is  most  rapid  on  the  hillsides  which  border  river 
valleys,  because  of  the  removal  of  waste  by  the  rivers.  Away 
from  the  streams  the  denudation  of  the  country  is  much  slower, 
because  the  waste  is  less  readily  removed.  Those  points  will 
longest  remain  standing  above  the  general  level  which  are  com- 
posed of  the  hardest  rocks  and  are  farthest  removed  from  the 
principal  lines  of  drainage. 

The  subaerial  agencies  act  with  the  greater  efficiency  the  more 
elevated  the  region  upon  which  they  operate.  Consequently,  so 
long  as  the  region  be  not  again  elevated,  denudation  operates  at 
a  continually  diminishing  rate.  The  strong  relief  of  hill  and 
valley  is  carved  out  with  comparative  rapidity,  but  the  more 
nearly  the  country  is  reduced  to  base-level,  the  more  slowly  does 
degradation  proceed,  and  the  final  stages  of  base-levelling  must  be 
exceedingly  slow.  Nevertheless,  if  no  renewed  upheaval  takes 
place,  the  loftiest  and  most  rugged  land  surface  must  be  eventu- 
ally cut  down  to  that  level.  The  universal  and  permanent  base- 
level  is,  of  course,  the  sea;  but  other  local  and  temporary 
base-levels  may  for  a  time  control  the  development  of  certain 
areas.  Tributaries  cannot  cut  below  the  main  stream  into  which 
they  flow ;  a  lake  forms  the  base-level  for  the  streams  which  sup- 
ply it,  until  the  lake  is  removed  by  draining  away  or  being  filled 
with  sediment.  Regions,  like  the  Great  Basin,  whose  drainage 
finds  no  outlet,  may  have  base-levels  either  above  or  below  the 
level  of  the  sea;  e.g.  the  surface  of  the  Dead  Sea  of  Palestine  is 
1308  feet  below  the  Mediterranean. 

It  is  perhaps  a  question  whether  any  large  region  has  ever 
remained  stationary  (with  reference  to  the  sea-level)  for  a  suffi- 
ciently long  time  to  be  absolutely  base-levelled.  On  the  other 
hand,  there  is  abundant  evidence  to  show  that  such  areas  have 


310  LAND    SCULPTURE 

been  worn  down  to  a  low-lying,  featureless  surface,  with  only 
occasional  low  protuberances  rising  above  the  general  level. 
Such  a  surface  is  called  a  peneplain,  and  represents  what  is  usu- 
ally the  final  stage  of  a  cycle  of  denudation.  Here  and  there  an 
isolated  peak  may  remain  high  enough  to  deserve  the  name  of 
mountain,  which  owes  its  preservation  to  the  exceptionally  resist- 
ant nature  of  the  rocks  of  which  it  is  composed,  or  to  its  excep- 
tionally favourable  position  with  reference  to  the  drainage  lines. 
A  renewed  upheaval  of  the  peneplain  will  begin  another  cycle 
of  denudation,  revivifying  and  rejuvenating  all  the  destructive 
agencies,  and  valleys  and  hills  will  be  carved  out  of  the  approxi- 
mately level  surface.  In  a  peneplain  dissected  by  the  revived 
streams  the  sky  line  of  the  ridges  is  notably  even,  and  all  the 
heights  rise  to  nearly  the  same  level.  Differences  of  level  are, 
however,  frequently  produced  by  a  warping  process,  which  may 
accompany  the  upheaval,  raising  some  portions  of  the  peneplain 
to  greater  heights  than  others.  Excellent  examples  of  reelevated 
and  subsequently  dissected  peneplains  are  the  uplands  of  southern 
New  England  and  the  highlands  of  New  Jersey. 

One  agent  of  subaerial  denudation  has  such  a  characteristic 
and  peculiar  method  of  work  that  it  requires  a  few  words  of 
separate  consideration.  This  agent  is  the  glacier.  In  Chapter 
VI  these  peculiarities  were  described,  and  it  will  suffice  here  to 
recall  them  briefly.  A  glacier,  in  those  parts  of  its  course  where 
deposition  does  not  occur,  sweeps  away  whatever  previous  accu- 
mulations of  soil  and  loose  de"bris  it  may  encounter,  laying  bare  the 
rock.  The  rocks,  hard  and  soft,  are  ground  down,  marked  with 
long  parallel  scorings,  grooved,  and  polished.  The  valley  in  which 
an  Alpine  glacier  flows  was  made  in  the  first  instance  by  the 
atmosphere  and  running  water,  but  the  moving  ice  has  modified 
it  in  several  particulars.  A  glacial  valley  is  widened,  losing  the 
V-shape  and  taking  on  more  the  shape  of  a  U.  The  longitudinal 
slope  of  the  valley  is  less  continuous  than  in  one  made  entirely  by 
water,  being  broken  by  ridges  or  escarpments  of  polished  rock, 
behind  which  are  depressions.  These  depressions  may  be  Con- 
verted into  lakes  when  the  glacier  has  retreated,  but  it  is  still  a 


GLACIERS  3 1 1 

question  how  far  a  glacier  is  able  to  excavate  solid  rock.  The 
fjords  of  Norway  are  glaciated  valleys  which  have  been  invaded 
by  the  sea. 

An  equally  characteristic  kind  of  topography  is  due  to  the 
constructive  work  of  glaciers.  The  terminal  moraine  of  a  valley 
glacier  or  of  the  lobes  and  tongues  given  off  by  an  ice-sheet, 
surrounds  the  end  of  the  ice,  with  its  concave  slope  directed  up 
the  valley,  thus  forming  a  more  or  less  crescentic  dam.  A  glacier 
retreating,  but  with  stationary  pauses,  builds  up  one  of  these 
moraines  at  each  halt,  or  if  the  retreat  be  rapid  and  continuous, 
the  material  is  spread  over  the  abandoned  ground  and  is  fre- 
quently worked  over  by  the  waters  derived  from  the  melting  ice 
into  stratified  deposits.  The  morainic  dams  often  pond  back 
waters  into  lakes.  Kettle  moraines,  which  have  deep  conical 
depressions  in  them,  are  believed  to  be  due  to  the  isolation  of 
masses  of  debris-covered  ice,  left  behind  by  the  retreating  glacier, 
and  the  subsequent  melting  of  these  isolated  masses  has  formed 
the  depressions.  To  what  extent  material  can  gather  beneath  a 
moving  mass  of  ice  is  still  an  open  question,  but  the  vanished  ice- 
sheets  have  left  over  much  of  their  former  courses  great  masses  of 
drift,  spread  like  a  mantle  over  the  ground  and  filling  up  the 
valleys.  In  the  northern  United  States  countless  stream  valleys 
have  thus  been  obliterated. 

In  considering  glacial  topography,  then,  we  have  to  deal  with 
the  work  of  erosion  and  of  deposition  by  the  ice,  each  of  which 
produces  effects  peculiar  to  itself.  In  the  central  zone  of  the  ice- 
sheet,  where  the  ice  remained  longest,  it  had  its  maximum  thick- 
ness and  destructive  efficiency.  Here  the  principal  work  is  that 
of  erosion,  and  when  the  ice  has  retreated,  we  find  great  areas  of 
naked,  striated,  and  polished  rocks,  abounding  in  roches  moutonnees 
and  in  lake-filled  rock  basins.  In  the  peripheral  zone  of  the  ice- 
sheet,  the  ice  was  thinner,  more  sluggish  in  movement,  was  sub- 
ject to  episodes  of  advance  and  retreat,  and  did  not  remain  for  so 
long  a  time.  Here  the  work  was  prevailingly  that  of  deposition, 
and  the  resulting  topography  has  little  relief,  and  that  relief  is 
very  irregular.  Sheets  of  drift,  morainic  mounds  and  dams,  which 


312  LAND   SCULPTURE 

enclose  small  lakes  or  marshes,  erratic  blocks,  and  quantities  of 
water-worked  and  more  or  less  stratified  drift  are  the  character- 
istic features.  The  confused  and  irregular,  but  low  relief  which 
marks  the  outer  zone  of  the  ice,  is  generally  succeeded  by  a  plain 
of  sand  or  gravel,  the  overwash  plain,  produced  by  the  debris- 
laden  waters  which  escaped  from  the  front  of  the  ice. 

Kames,  Eskers,  and  Drumlins.  —  These  are  peculiar  forms  of 
glacial  accumulations  which  are  found  in  the  peripheral  zone. 
Kames  are  hillocks  or  short  ridges  of  stratified  drift,  formed  by 
the  deposition  of  materials  from  subglacial  streams  as  they  escape 
from  under  the  margin  of  the  ice.  Eskers  or  Asar  are  long, 
winding  ridges  of  sand  and  gravel,  which  may  have  considerable 
height  and  which  follow  the  general  direction  of  the  moving  ice. 
Several  eskers  may  join  one  another,  just  like  a  stream  and  its 
tributaries.  The  accepted  explanation  of  these  ridges  is,  that 
they  mark  the  beds  of  streams  which  flowed  upon  or  under  the 
ice,  near  its  edge  ;  the  sand  and  gravel  were  laid  down  in  channels 
or  tunnels  in  the  ice  and  thus  were  prevented  from  spreading  out 
into  a  flat  sheet.  When  the  ice  retreated,  the  stream  deposits 
were  left  standing  as  ridges.  Kames  and  eskers  are  common  in 
glaciated  regions,  and  are  well  displayed  in  New  England  and  in 
central  New  York.  The  latter  are  probably  forming  under  the 
Malaspina  glacier  now.  (See  p.  157.)  Drumlins  are  elliptical 
hills  or  mounds  (sometimes  200  feet  high),  which  are  arranged 
in  lines  coincident  with  the  direction  taken  by  the  moving  ice. 
Drumlins  are  not  at  all  or  only  partially  stratified,  and  were  formed 
by  the  combined  action  of  ice  and  water  or  by  ice  alone  near 
the  margin  of  the  ice-sheet.  Thousands  of  them  exist  in  the 
northern  United  States,  especially  in  New  England,  Wisconsin, 
and  Minnesota. 

A  glaciated  topography  is  one  marked  by  rounded,  flowing  out- 
lines, in  contrast  to  the  craggy  hills  of  regions  which  have  never 
been  smoothed  by  ice.  In  one  way  or  another,  a  glacier  leaves 
many  depressions  and  basins  in  its  tract,  which,  when  the  ice  has 
retreated,  become  filled  with  water  and  form  lakes,  A  glaciated 
region  is  preeminently  a  region  of  lakes, 


ARRANGEMENT  OF   ROCKS  313 

TOPOGRAPHY  CONDITIONED  BY  THE   ARRANGEMENT  OF  ROCKS 

While  the  final  effect  of  the  subaerial  denuding  agencies  is  to 
sweep  away  all  relief,  and  to  cut  the  land  surface  down  to  low- 
lying  base-levels  or  peneplains,  yet  in  the  process  great  irregu- 
larities are  produced  by  the  more  rapid  removal  of  some  parts 
than  of  others.  The  topographical  forms  generated  by  this  dif- 
ferential erosion  vary  much  according  to  circumstances.  We 
have  already  considered  some  of  these  differences  with  regard 
to  the  agencies  which  have  produced  them.  Now  we  have  to 
examine  the  differences  with  a  view  of  learning  how  topographi- 
cal forms  are  determined  by  the  character  and  arrangement  of 
the  rocks  which  are  undergoing  degradation. 

When  a  peneplain  or  plain  of  marine  denudation  is  lifted  high 
above  sea-level,  without  folding  or  steep  tilting  of  the  strata, 
streams  are  soon  established  upon  the  new  land,  and  proceed 
to  cut  deep  trenches  across  the  plateau,  which  are  gradually 
widened  out  under  the  influence  of  weathering,  and  the  arrange- 
ment of  hard  and  soft  rocks  finds  expression  in  the  resulting 
forms.  If  the  surface  layers  resist  weathering,  the  plateau  will  be 
gradually  dissected  into  flat-topped  mesas  and  table-mountains; 
while  if  the  whole  mass  of  rocks  be  easily  destructible,  they 
weather  down  into  dome-shaped  and  rounded  hills,  which  are 
smallest  at  the  top,  the  part  longest  exposed  to  weathering.  The 
wild  and  grotesque  scenery  of  the  Western  bad  lands,  with  their 
chaos  of  peaks,  ridges,  mesas,  and  buttes,  is  merely  the  result  of 
the  differential  weathering  of  horizontal  strata,  some  beds  and 
parts  of  beds  yielding  more  readily  than  others.  If  a  series  of 
more  resistant  beds  underlies  a  mass  of  softer  strata,  a  change  in 
the  topographical  forms  will  occur  when  the  underlying  harder 
rocks  are  partially  exposed.  In  the  soft  rocks  the  valley  sides 
have  gentle  slopes,  but  when  the  harder  mass  is  penetrated,  the 
slopes  become  steep,  or  even  vertical.  When  hard  and  soft 
-strata  alternate  in  a  valley  wall,  the  harder  beds  form  cliffs.  This 
is  accomplished  by  cutting  away  the  softer  beds  and  thus  under- 
mining the  harder  ones,  until  the  latter  can  no  longer  support 


314  LAND    SCULPTURE 

their  own  weight,  and  masses  fall  from  the  face  of  the  cliff,  thus 
maintaining  the  verticality.  The  talus  blocks  form  a  slope,  con- 
necting the  successive  cliffs  by  gentler  inclines.  The  Uinta 
Mountains  in  northern  Utah  are  formed  by  a  great  anticlinal 
arch,  so  broad  and  gently  curved  that  in  a  given  section  the 
strata  appear  almost  horizontal.  Out  of  these  immensely  thick 
and  nearly  level  masses  the  atmospheric  denuding  agencies  have 
carved  an  infinite  and  most  picturesque  variety  of  peaks,  pin- 
nacles, columns,  and  amphitheatres,  while  the  streams  have  cut 
profound  and  gloomy  canons.  Vast  talus  slopes  remain  to  indi- 
cate the  amount  of  destruction. 

Inclined  or  tilted  strata  give  rise  to  a  different  class  of  topo- 
graphical forms.  If,  as  is  generally  the  case,  harder  and  softer  strata 
alternate,  the  latter  will  be  swept  away  more  rapidly  than  the 
former,  which  are  left  standing  as  ridges  or  cliffs,  the  height  and 
steepness  of  which  are  determined  by  the  thickness  and  inclina- 
tion of  the  more  resistant  rocks.  In  case  the  strata  are  steeply 
inclined,  a  succession  of  hard  beds  alternating  with  soft  will 
give  rise  to  a  series  of  ridges  and  valleys,  the  slopes  of  which 
depend  upon  the  angle  of  dip.  If  the  beds  are  standing  in  a, 
vertical  position,  the  two  slopes  of  each  ridge  will  be  nearly  equal, 
the  hard  strata  forming  the  backbone  of  the  ridge  and  the  soften 
ones  the  sloping  sides.  As  the  inclination  departs  from  verticality, 
the  more  unequal  do  the  two  slopes  of  each  ridge  become  the 
longer  and  gentler  one  being  in  the  direction  of  the  dip.  Ridges 
and  valleys  of  this  class  are  beautifully  exemplified  in  the  Appa- 
lachian Mountains.  Figure  25  shows  Kittatinny  Mountain,  through 
which  the  Delaware  -River  has  cut  the  famous  Water  Gap ;  the 
crest  of  the  ridge  is  formed  by  very  hard  and  indestructible 
sandstones  and  conglomerates,  while  the  valley  is  in  slates. 

In  gently  inclined  strata  the  abruptly  truncated  and  cliff-like  out- 
crops of  the  hard  strata  are  called  escarpments,  and  follow,  with  some 
irregularities  and  sinuosities,  the  strike  of  the  beds.  Whether  the 
general  course  of  the  escarpment  shall  be  straight  or  curved  will, 
therefore,  be  determined  by  the  constancy  or  change  in  the  direc- 
tion of  the  dip  ;  for,  as  we  have  already  learned,  the  strike  changes 


ESCARPMENTS 


315 


3l6  LAND    SCULPTURE 

with  the  dip,  always  keeping  at  right  angles  to  the  latter.  The 
upper  surface  of  the  gently  inclined  hard  stratum  may  be  com- 
pletely exposed  by  the  stripping  away  of  the  softer  overlying 
mass,  and  then  the  slope  of  the  ground  is  the  same  as  that  of  the 
resistant  stratum  and  is  called  a  dip  slope.  A  series  of  gently 
inclined  strata,  made  up  of  alternating  harder  and  softer  beds, 
will  thus  give  rise  to  parallel  ridges  and  valleys,  or  escarpments 
and  dip  slopes,  according  to  the  completeness  with  which  the 
softer  beds  are  removed  and  the  harder  ones  exposed.  A  mag- 
nificent example  of  such  escarpments  and  slopes  is  displayed  in 
the  high  plateaus  of  Utah  and  Arizona,  where  the  dip  slopes  are 
from  20  to  60  miles  broad  and  the  escarpments  1500  to  2000  feet 
high.  The  amount  of  denudation  involved  in  the  production  of 
these  vast  amphitheatres  staggers  belief,  though  there  is  no  escape 
from  the  enormous  figures. 

Under  the  influence  of  denudation  escarpments  are  continually 
though  slowly  receding,  being  cut  back  in  the  direction  of  the  dip. 
Rain  and  frost  act  directly  upon  the  hard  beds,  but  work  more 
effectively  by  cutting  away  the  softer  beds  below  and  thus  under- 
mining the  hard  strata,  causing  them  to  fall.  The  fallen  masses 
are  gradually  disintegrated  in  their  turn  and  washed  away  into 
the  water-courses.  The  escarpments  may  follow  a  relatively 
straight  or  a  very  sinuous  course.  Sinuosities,  when  present,  are 
commonly  due  to  the  presence  of  springs,  which  undermine  the 
escarpments  and,  by  the  recession  of  their  heads,  excavate  the  line 
of  cliffs  into  bays  and  amphitheatres.  A  sinuous  escarpment  is  more 
rapidly  cut  back  than  a  straight  one,  because,  in  addition  to  the 
cooperation  of  the  springs,  it  offers  a  larger  surface  to  the  attack 
of  the  destructive  agencies.  Every  step  in  the  recession  of  an 
escarpment  lowers  the  ridge  and  brings  it  nearer  to  base-level, 
because  the  direction  of  retreat  follows  the  line  of  dip,  which 
carries  the  beds  down  to  base-level  with  a  rapidity  determined 
by  the  angle  of  dip.  A  steeply  inclined  bed  needs  to  be  cut  back 
only  a  short  distance,  when  it  will  be  reduced  to  base-level, 
whereas  a  bed  dipping  very  gently  remains  above  base-level  for 
very  long  distances.  Of  course  the  general  elevation  of  the  whole 


TRANSVERSE  VALLEYS 


317 


region  above  base-level  is  also  an  important  factor  in  determining 
the  amount  of  work  to  be  done. 

For  reasons  that  will  appear  later,  we  assume  that  when  denu- 
dation began  its  work  upon  a  region  of  inclined  strata,  that  region 
was  a  sloping  plain,  or  peneplain,  formed  by  the  outcropping  edges 


FlG.  133.  —  Mesa  and  round-topped  buttes,  exhibiting  unconformity  and  change 
in  character  of  the  strata.     Bad  lands  of  South  Dakota. 

of  the  strata.  The  first  lines  of  drainage  established  would  neces- 
sarily follow  this  slope,  and  the  first  valley  or  valleys  cut  would  be 
across  the  strike  of  the  beds,  trenching  both  hard  and  soft  beds. 
Such  valleys  are  called  transverse  and  the  streams  which  flow  in 
them,  transverse  streams.  A  second  series  of  valleys  will  be  exca- 
vated along  the  strike  of  the  softer  beds,  giving  longitudinal  val- 


318  LAND   SCULPTURE 

leys  and  streams.  In  such  a  longitudinal  valley,  following  the 
strike  of  a.  mass  of  soft  strata,  the  stream  which  occupies  it  will 
tend  to  flow  along  the  foot  of  the  escarpment  formed  by  the  out- 
crop of  hard  strata,  and  to  shift  its  course  laterally  in  the  direction 
of  the  dip,  cutting  away  the  soft  beds  in  which  it  flows,  and  under- 
mining the  hard  escarpment.  Such  a  stream  is  a  potent  agent  in 
causing  the  recession  of  the  escarpment  and  may  remove  large 
areas  of  both  hard  and  soft  strata. 

The  steep  ridges,  or  "  hog-backs,"  which  occur  among  the  foot- 
hills of  the  Rocky  Mountains  in  Colorado,  show  interesting  ex- 
amples of  streams  flowing  along  the  strike  of  inclined  strata, 
though  the  ridges  are  themselves  not  formed  quite  in  the  way 
already  described.  They  are  composed  of  the  steeply  dipping 
limbs  of  monoclinal  folds,  of  which  the  upper  horizontal  limbs 
have  been  removed  by  denudation  (Fig.  132). 

Folded  Strata.  —  A  region  of  folded  strata  is,  in  the  first  in- 
stance, thrown  into  a  series  of  ridges  and  valleys,  the  ridges  formed 
by  anticlines  and  the  valleys  by  synclines.  If  the  folding  be  of 
moderate  degree,  so  as  to  produce  undulations  of  sweeping  and 
gentle  curves,  the  tendency  of  denudation  is  to  reverse  the  original 
topography  and  convert  the  anticlines  into  valleys  and  the  syn- 
clines into  ridges.  This  apparently  paradoxical  result  is  found, 
when  examined,  to  be  natural  and  simple  enough.  The  crests  of 
newly  formed  anticlines  have  been  subjected  to  tensile  stresses 
which  open  the  joints  in  the  strata  and  render  them  an  easy 
prey  to  the  denuding  agents.  The  surface  of  the  synclines,  on 
the  contrary,  has  been  tightly  compressed,  and  their  joints  are 
closed  by  crowding.  Aside  from  this,  another  factor  tends  to  pro- 
duce the  same  result.  In  a  folded  series  of  alternating  harder 
and  softer  beds  denudation  is  most  rapid  on  the  exposed  anticlines, 
and  in  them  the  hard  strata  are  first  reached  and  cut  through. 
When  an  underlying  mass  of  soft  strata  is  reached,  they  are  rapidly 
trenched  into  valleys  which  may  soon  be  excavated  below  the  level 
of  the  synclinal  troughs. 

If  the  folds  originally  made  by  the  force  of  lateral  compression 
be  steep  and  high,  as  in  mountain  ranges,  the  anticlines  persist 


OUTLIER  AND   INLIER  319 

longer  as  ridges,  but  the  wearing  away  of  their  summits  gives 
rise  to  subordinate  ridges  and  valleys  within  the  limits  of  each 
anticlinal  arch.  Here  also  the  ridges  are  the  outcropping 
harder  beds,  and  the  valleys  are  cut  in  the  softer  ones.  Even 
in  mountain  ranges  denudation  may  reverse  the  original  struct 
ural  topography  and  give  rise  to  anticlinal  valleys  and  synclinal 
mountains. 

If  a  region  of  folded  rocks  has  once  been  planed  down  to 
base-level  or  to  a  peneplain,  and  then  reelevated  and  subjected 
to  denudation,  the  resulting  topography  will  be  determined  by 
the  same  laws.  Indeed,  this  is  the  method  in  which  regions 
of  tilted  or  inclined  strata  are  produced,  for,  as  we  saw  in 
Chapter  XII  (p.  232),  inclined  beds  are  generally  parts  of  trun- 
cated folds.  In  such  regions  drainage  is  first  in  accordance 
with  the  slopes  of  the  planed  and  tilted  surface,  but  as  denuda- 
tion proceeds,  the  structure  and  arrangement  of  the  rocks  make 
themselves  felt,  and  bring  about  changes  and  adjustments  of  the 
drainage  to  the  structure,  as  will  be  more  fully  explained  in  the 
following  chapter. 

The  combined  action  of  the  displacement  and  dislocation  of 
the  strata,  on  the  one  hand,  and  of  denudation,  on  the  other,  often 
results  in  the  formation  of  disconnected  patches  of  rock  which, 
according  to  their  geological  relations,  are  called  outliers  and 
inliers.  An  outlier  is  an  isolated  mass  of  rock,  like  an  island, 
which  has  been  cut  off  by  denudation  from  its  former  connections. 
Outliers  are  sometimes  scores  of  miles  from  the  nearest  mass 
of  the  same  strata,  and  they  stand  as  monuments  which  show, 
partially  at  least,  the  former  extension  of  the  eroded  beds. 
An  outlier  rests  upon  the  underlying  strata,  and  when  viewed 
on  a  map,  which  brings  all  projections  down  to  one  plane,  is 
surrounded  by  beds  which  are  geologically  older  than  itself. 
Outliers  are  almost  always  composed  either  of  horizontal  strata, 
or  of  isolated  synclines,  for  an  isolated  anticline  would  soon  be 
swept  away  by  denudation,  the  dip  coinciding  with  the  slope  of 
the  surface,  an  arrangement  very  favourable  to  landslips  and  rapid 
erosion. 


32O  LAND   SCULPTURE 

An  inlier  is  produced  by  the  truncation  of  an  anticline  or  a 
dome,  exposing  an  area  of  a  lower  stratum  surrounded  by  strata 
which  are  geologically  above  it.  If  the  isolated  mass  be  brought 
to  the  surface  by  faulting,  it  is  called  a  faulted  outlier  or  inlier, 
according  as  it  is  surrounded  by  older  or  younger  beds.  A  faulted 
outlier  is  on  the  downthrow  side  of  the  dislocation  and  a  faulted 
inlier  on  the  upthrow  side. 


CHAPTER    XVIII 

ADJUSTMENT  OF   RIVERS 

RIVERS  are  among  the  most  powerful  of  the  agents  of  topo- 
graphical development,  and  it  is  important  to  understand  some- 
thing of  their  modes  of  change  and  adjustment.  These  changes 
are  sometimes  exceedingly  complex  and  puzzling,  for  rivers  do  the 
most  unexpected  things  in  what  seems  an  utterly  capricious  and 
whimsical  way.  We  often  see  rivers  breaching  hills  and  even  vast 
mountain  ranges,  cutting  their  way  through  enormous  obstacles, 
which  a  slight  deviation  from  their  course  would,  seemingly,  have 
enabled  them  to  avoid.  They  apparently  choose  the  difficult  and 
shun  the  easy  path.  The  general  explanation  of  these  paradoxi- 
cal results  is,  that  the  river  began  its  flow  when  the  topography 
was  entirely  different  from  its  present  state  of  development.  It 
is  this  fact  which  renders  the  rivers  such  valuable  aids  to  the 
geologist  in  his  attempts  to  reconstruct  the  past,  for  the  apparent 
whims  and  caprices  are  really  the  necessary  results  of  law. 

A  river  has  its  stages  of  development,  youth,  maturity,  and  old 
age,  just  as  has  a  land  surface,  each  stage  displaying  its  character- 
istic marks.  When  a  new  land  is  upheaved,  the  first  drainage 
lines  established  upon  it  are,  as  we  have  already  learned,  deter- 
mined entirely  by  the  slopes  of  the  new  surface  and  are  called 
consequent  streams.  In  its  earliest  stages  a  river  can  drain  its 
territory  or  basin  in  only  imperfect  fashion,  and  whatever  depres- 
sions exist  in  the  surface  of  the  new  land  are  filled  up  with  water 
and  form  lakes.  Tributaries  are  much  fewer  than  in  later  stages 
of  development ;  the  divides  between  the  tributaries  are  obscurely 
marked,  and  in  plains  these  divides  are  broad  areas,  not  lines. 
The  Red 'River  of  the  North  is  an  example  of  a  stream  in  a  very 
youthful  stage,  which  flows  across  the  level  floor  of  an  abandoned 
y  321 


322  ADJUSTMENT  OF   RIVERS 

lake.  In  this  plain  the  divides  between  the  streams  are  so  wide 
and  flat  that  water  gathers  on  them  after  heavy  rains,  having  no 
reason  to  flow  in  one  direction  rather  than  another. 

As  the  river  system  becomes  somewhat  older,  the  stream  chan- 
nels are  deepened,  the  larger  ones  being  cut  down  to  base-level, 
and  if  the  region  be  one  of  considerable  elevation,  deep  gorges 
and  canons  are  excavated.  If  the  streams  flow  across  strata  of 
different  hardness,  waterfalls  result  where  a  hard  ridge  crosses 
them,  but  in  the  main  stream  these  cascades  and  rapids  are 
ephemeral  and  soon  removed  by  the  stream's  wearing  down  the 
obstacle.  On  the  head-waters  of  streams,  however,  waterfalls  may 
persist  for  a  long  period.  The  river  valleys  are  widened  out  by 
atmospheric  denudation,  and  channels  are  formed  on  their  sloping 
sides,  which  gradually  grow  into  side  valleys.  The  lakes  are  for 
the  most  part  drained  or  silted  up,  only  the  more  important  and 
deeper  ones  remaining,  while  the  system  of  tributary  streams  and 
rills  is  greatly  expanded.  A  mature  river  system  is  characterized 
by  the  complete  development  of  its  tributaries  and  drainage,  so 
that  every  part  of  its  basin  is  reached  by  the  ramifying  channels. 
The  waterfalls  have  disappeared,  except  near  the  stream-heads, 
and  the  stream  channels  have  sought  out  and  utilized  every 
weakness  in  the  strata,  adjusting  themselves  to  the  structure  of 
the  rocks  and  the  alternations  of  hard  and  soft  beds. 

The  complete  network  of  streams  has  enlarged  the  valley 
surfaces,  which  increases  the  rate  of  destruction  and  brings  to 
the  river  a  greater  load  of  sediment  to  carry.  In  maturity  the 
river  receives  its  maximum  load,  sometimes  so  great  that  the  lower 
reaches  of  the  main  stream  are  unable  to  transport  it  all,  and 
spread  the  excess  out  over  the  flood  plain.  The  channel  of  an 
overloaded  stream  may  be  so  raised  and  banked  in  by  its  own 
deposits,  that  some  of  the  tributaries  are  deflected  and  made  to 
run  for  some  distance  parallel  to  the  main  stream,  perhaps  even 
reaching  the  sea  independently.  An  example  of  this  is  the  Loup 
Fork  of  the  Platte  in  Nebraska.  "  The  Platte  (lows  there  upon  a 
ridge  of  its  own  creation.  The  Loup  comes  down  into  its  valley 
and  flows  parallel  with  it  for  many  miles."  (Gannett.) 


DRAINAGE   OF   FOLDED   AREAS  323 

The  final  stages  of  river  development  are  reached  when  the 
base-level  is  attained,  and  the  drainage  basin  reduced  to  a  pene- 
plain by  the  combined  action  of  the  streams  and  weathering.  The 
flood-plain  deposits  may  now  be  partially  or  completely  removed, 
for  the  main  trunk  no  longer  receives  an  excessive  load,  and 
hence  it  is  able  to  carry  away  some  of  that  sediment  which  it  had 
previously  deposited.  With  its  drainage  basin  smoothed  down 
into  a  peneplain,  the  river's  work  is  done ;  it  has  reached  old  age. 
The  course  of  river  evolution  above  described  is  the  ideal  cycle 
of  development  which,  however,  may  be  and  generally  is  inter- 
rupted by  diastrophic  movements.  An  elevation  of  the  region 
may  simply  rejuvenate  the  streams  and  start  them  afresh  upon  a 
career  of  wearing  down  the  land.  But  if  accompanied  by  exten- 
sive warping  or  folding  of  the  rocks,  the  drainage  system  of  the 
entire  region  may  be  revolutionized.  A  depression  of  the  region 
will  have  the  contrary  effect,  checking  or  stopping  the  work  in 
which  the  streams  were  engaged,  drowning  their  lower  reaches 
and  converting  them  into  estuaries.  A  lowered  land  surface  has 
less  material  to  lose  before  it  is  reduced  to  base-level,  but  the 
work  of  denudation  is  accomplished  more  slowly. 

In  a  newly  formed  mountain  region  the  drainage  system  is  at 
first  consequent  upon  the  slopes  produced  by  folding,  the  principal 
streams  flowing  in  the  synclinal  troughs  and  passing  from  one 
syncline  to  another  at  the  points  where,  owing  to  the  descending 
pitch  of  the  folds,  the  anticlines  are  lowest.  The  principal  valleys 
are  thus  longitudinal.  Such  a  system  of  drainage  is  exemplified  in 
the  Jura  Mountains  of  Switzerland,  a  region  where  the  topography 
is  still  dominated  almost  completely  by  the  regular  folds  which 
form  anticlinal  ridges  and  synclinal  valleys.  In  very  ancient 
regions  of  folded  rocks,  on  the  other  hand,  the  original  longi- 
tudinal valleys  may  become  altogether  insignificant  in  comparison 
with  the  transverse  valleys  excavated  by  the  streams. 

When  it  was  first  suggested  that  rivers  had  cut  their  own  valleys 
and  had  not  merely  taken  possession  of  ready-made  trenches,  it 
was  objected  that  such  an  explanation  required  many  streams  to 
begin  their  course  by  flowing  up  hill.  It  is  very  common  to  find 


324  ADJUSTMENT  OF  RIVERS 

a  stream  flowing  across  a  region,  cutting  its  way  through  ridge 
after  ridge,  instead  of  following  the  easy  path  of  the  longitudinal 
valleys.  This  is  just  what  the  principal  streams  of  the  northern 
Appalachians,  such  as  the  Delaware,  the  Susquehanna,  and  the 
Potomac  have  done,  and  at  first  sight,  their  course  is  very  difficult 
to  explain.  Without  going  very  far  back  into  the  history  of  these 
mountains,  we  may  simply  state  that  the  ridges  through  which  the 
rivers  named  have  cut  are  the  remnants  of  a  reelevated  and  dis- 
sected peneplain,  across  which  the  streams  flowed  to  the  sea,  cut- 
ting transverse  valleys  that  were  rapidly  deepened  into  gorges. 
On  the  soft  strata  longitudinal  valleys  were  opened  out  which, 
however,  were  formed  after  the  transverse  streams  and  could  not 
be  deepened  faster  than  they,  because  the  main  stream  flowing  in 
each  transverse  valley  gave  a  temporary  base-level  for  the  tribu- 
taries flowing  in  the  longitudinal  valleys.  The  hard  beds  were 
sawed  through  by  the  descending  streams,  but  elsewhere  these 
beds  stood  up  as  ridges,  and  thus  the  ridges  are  also  younger  than 
the  streams.  The  mystery  disappears  at  once,  if  we  simply  remem- 
ber that  the  transverse  streams  began  their  flow  upon  a  sloping 
plain  above  which  the  present  ridges  did  not  project. 

Antecedent  Rivers.  —  Another  way  in  which  rivers  have  been 
enabled  to  cut  their  way  through  opposing  ranges  of  hills  and 
even  mountains,  is  by  occupying  the  district  before  the  hills  or 
mountains  were  made.  Such  streams  are  called  antecedent  and 
are  defined  as  "  those  that  during  and  for  a  time  after  a  disturb- 
ance of  their  drainage  area  maintain  the  courses  that  they  had 
taken  before  the  disturbance."  (Davis.)  The  simplest  case  of 
antecedent  drainage  is  where  an  area  is  uplifted  without  deforma- 
tion and  without  changing  the  direction  of  the  slopes.  Under 
such  circumstances  all  the  streams  retain  their  old  channels,  and 
simply  gain  renewed  power  to  cut  them  into  deeper  trenches, 
down  to  the  new  base-level.  Such  streams  are  said  to  be  revived. 
Even  if  the  upheaval  be  accompanied  by  folding  or  deformation, 
one  or  more  of  the  streams  may  persist  in  its  ancient  course, 
provided  the  folding  be  very  slow  and  gradual,  so  that  the  river  is 
able  to  cut  down  through  the  obstacles  which  are  raised  athwart 


SUPERIMPOSED    RIVERS  325 

its  course.  A  revolving  saw  cuts  its  way  through  a  log  which  is 
pushed  against  it,  so  the  river  cuts  its  way  through  the  rising 
barrier.  If  the  latter  be  raised  faster  than  the  river  can  cut,  then 
the  stream  will  be  dammed  back  into  a  lake,  or  will  be  diverted 
to  a  new  course. 

A  famous  example  of  what  many  authorities  believe  to  be  an 
antecedent  stream  is  the  Green  River  in  Wyoming  and  Utah. 
Entering  from  the  north,  the  river  cuts  its  way  in  a  winding  course 
through  the  great  mountain  barrier  of  the  Uintas  in  a  remarkable 
series  of  canons,  although  by  turning  a  short  distance  to  the  east- 
ward, it  might  have  found  a  path  several  thousand  feet  lower  than 
the  one  which  it  has  chosen.  Even  more  extraordinary  is  the 
Yampa,  which  flows  in  from  the  east,  and  though  a  slight  southerly 
deflection  would  have  brought  it  to  the  Green  on  the  plains  south  of 
the  mountains,  it  cuts  its  profound  gorge  along  the  axis  of  the  uplift, 
meeting  the  Green  in  the  heart  of  the  range.  The  strange  be- 
haviour of  these  two  streams  is  usually  explained  by  regarding  them 
as  antecedent,  occupying  nearly  the  same  channels  as  they  had 
before  the  mountains  were  upheaved.  The  uplift  was  so  gradual 
that  the  rivers  cut  through  the  barriers  as  fast  as  the  latter  were 
raised.  This  explanation  is  not  accepted  by  all  the  observers 
who  have  examined  the  region,  some  of  whom  explain  the  phe- 
nomena by  the  theory  of  superimposed  drainage,  described  in  the 
following  section.  Several  rivers  in  the  Alps  and  Himalayas, 
which  rise  in  the  inner  part  of  the  ranges  and  cut  their  way  out 
through  deep  chasms,  are  believed  to  be  antecedent. 

Superimposed  Rivers.  —  A  region  newly  upheaved  from  the  sea 
is  covered  to  a  greater  or  less  depth  with  marine  deposits  which 
may  lie  unconformably  upon  a  foundation  of  older  rocks  of  en- 
tirely different  character,  arrangement,  and  structure.  The  system 
of  drainage  established  upon  the  new  land  is  at  first  consequent 
upon  the  initial  slopes  of  the  region.  As  the  streams  cut  their 
trenches  through  the  overlying  mantle  of  newer  strata,  they  en- 
counter the  older  rocks  below,  first  laying  bare  the  higher  ridges  of 
the  latter,  which  will  cause  waterfalls  and  rapids.  The  upper 
Mississippi  has  in  many  places  excavated  its  channel  through  the 


326  ADJUSTMENT   OF   RIVERS 

surface  sheet  of  glacial  drift  and  is  now  engaged  in  eroding  the 
ancient  crystalline  rocks  which  the  drift  had  covered.  When  the 
stream  has  everywhere  cut  through  the  newer  rocks,  its  course 
will  be  seen  to  have  no  relation  to  the  structure  of  the  older  rocks 
which  it  is  now  trenching.  If,  as  frequently  has  happened,  denuda- 
tion has  stripped  away  almost  all  the  newer  strata,  the  drainage  of 
the  country  seems  to  be  quite  inexplicable  and  to  be  arranged  with- 
out any  reference  to  the  structure  of  the  rocks  across  which  the 
streams  flow.  Such  a  system  of  drainage  is  said  to  be  superimposed, 
inherited,  or  epigenetic. 

Subsequent  Streams.  —  As  a  river  system  approaches  maturity, 
and  as  the  drainage  of  the  area  becomes  more  complete,  it  will 
increase  the  number  of  its  branches.  Those  branches  which 
were  not  at  all  represented  in  the  youthful  stages  of  the  system, 
and  are  opened  out  along  lines  of  yielding  rocks,  are  called  sub- 
sequent, and  all  streams  will  develop  more  or  fewer  of  such 
branches  as  they  advance  to  maturity. 

Adjustment  of  Streams.  —  However  the  streams  of  a  district 
may  have  been  established  in  the  first  instance,  whether  they 
were  consequent,  antecedent,  or  superimposed,  they  are  liable  to 
changes  more  or  less  profound  and  far-reaching.  These  changes, 
which  belong  to  the  normal  development  of  the  drainage  system 
and  are  not  dependent  upon  diastrophism,  are  due  to  adjustment 
of  the  streams  to  the  rock  structure  of  the  district,  the  streams 
searching  out  the  lines  of  weakness  and  least  resistance,  and 
everywhere  taking  the  easiest  path  to  their  destination.  The  up- 
stream extension  of  branches  and  the  shifting  of  the  divides  result 
in  the  capture  of  streams,  or  parts  of  such,  by  others  more  favour- 
ably situated,  one  master  stream  gradually  absorbing  many  smaller 
ones  which  had  originally  been  independent. 

A  divide,  or  water-parting,  between  two  streams  is  gradually 
shifted  by  the  lengthening  of  the  more  favourably  situated  stream, 
or  of  one  of  its  subsequent  branches.  This  more  favourable  situa- 
tion may  be  because  it  has  a  shorter  course  and  greater  fall,  giving 
a  swifter  flow,  or  because  it  flows  at  a  lower  level,  giving  greater  fall 
to  its  tributaries,  or  because  its  course  is  through  soft  and  easily 


STREAM   CAPTURE  327 

eroded  rocks,  while  its  rival  is  embarrassed  by  hard  rocks  and 
ledges.  Another  favourable  circumstance  which  may  decide  be- 
tween streams  otherwise  equal  is  given  by  the  attitude  of  the  strata. 
In  regions  of  inclined  strata,  as  we  have  already  learned,  the 
escarpments  formed  by  outcropping  ledges  of  harder  rocks  tend 
to  migrate  in  the  direction  of  the  dip.  As  such  escarpments 
frequently  form  divides  between  minor  streams,  the  stream  towards 
which  the  escarpment  migrates  will  be  at  a  disadvantage.  This 
shifting  of  divides  is  a  very  slow  process,  but  after  a  long  time  of 
insidious  advance  the  actual  capture  and  diversion  of  part  of  a 
stream  may  be  quite  suddenly  effected. 

Stream  capture  may  be  effected  in  a  great  variety  of  ways,  but 
a  few  examples  must  suffice.  We  may,  in  the  first  place,  suppose 
two  neighbouring  streams  following  roughly  parallel  courses,  but, 
owing  to  the  original  conformation  of  the  region,  flowing  at 
different  levels.  The  stream  that  flows  at  the  lower  level  will 
allow  greater  fall  to  its  tributaries,  which  will  thus  work  upward 
more  rapidly.  One  of  these  tributaries  will  eventually  work 
its  way  through  the  divide  and  tap  the  rival  stream,  all  of  whose 
waters  above  the  point  of  tapping  will  be  diverted  to  the  main 
stream  which  flows  at  the  lower  level. 

Another  method  of  stream  capture  is  well  illustrated  by  the 
Delaware,  the  Potomac,  and  other  transverse  rivers  which  have 
cut  deep  gorges  through  the  Appalachian  ridges.  Suppose  two 
parallel  transverse  streams  flowing  across  a  gently  sloping  pene- 
plain which  is  composed  of  tilted  rocks  of  different  degrees  of 
hardness.  In  the  manner  already  explained  (p.  317)  these  streams 
cut  gorges  through  the  ridges  of  hard  rock,  while  longitudinal  val- 
leys are  worn  out  along  the  strike  of  softer  strata,  which  valleys  are 
occupied  by  tributaries  of  the  transverse  streams.  If  one  of  the 
two  transverse  streams  be  considerably  larger  than  the  other  it  will 
saw  its  way  through  the  hard  ridges  at  a  correspondingly  faster  rate 
and  establish  a  lower  base-level  for  its  tributaries.  One  of  th 
tributaries  with  its  more  rapid  fall  will  be  thus  enabled  to  shift  its 
divide  at  the  expense  of  a  branch  of  the  rival  transverse  stream, 
capture  it,  and  by  reversing  the  direction  of  its  flow  draw  off  the 


328  ADJUSTMENT  OF   RIVERS 

waters  of  the  smaller  main  stream  above  the  point  where  its  capt- 
ured tributary  entered  it.  Or,  a  tributary  of  the  larger  main  stream 
may  push  its  way  up  a  longitudinal  valley  until  it  taps  and  diverts 
the  smaller  transverse  stream  without  the  intermediation  of  any 
tributary  of  the  latter.  Examples  of  both  of  these  varieties  of 
capture  may  be  found  among  the  Appalachian  rivers  ;  an  excellent 
illustration  of  the  latter  method  is  given  by  the  Potomac  and 
Shenandoah. 

When  the  Potomac  was  beginning  to  cut  its  gap  through  the 
Blue  Ridge  at  Harper's  Ferry,  a  smaller  stream,  Beaverdam  Creek, 
was  cutting  a  similar  gorge  through  the  same  ridge  a  few  miles  to 
the  south.  The  Shenandoah  was  then  a  young  and  short  tributary 
of  the  Potomac,  which  it  entered  from  the  south,  flowing  through  the 
longitudinal  valley  which  was  opening  along  the  strike  of  the  softer 
strata  to  the  west  of  the  Blue  Ridge.  As  the  Potomac  is  much 
larger  than  Beaverdam  Creek,  it  cut  its  gap  much  more  rapidly, 
thus  giving  a  steep  and  swift  course  to  the  Shenandoah.  The 
latter  pushed  its  way  up  the  longitudinal  valley  until  it  tapped 
Beaverdam  Creek  and  captured  its  upper  course,  diverting  its 
waters  to  the  Potomac.  Beaverdam  Creek  no  longer  flowed 
through  the  gorge  which  it  had  cut  in  the  Blue  Ridge  and  which 
was  thus  abandoned  and  became  a  "  wind  gap,"  the  beheaded 
Beaverdam  now  rising  to  the  eastward  of  the  abandoned  gorge. 
This  gorge  is  known  as  Snickers  Gap.  The  great  number  of  wind 
gaps  in  the  Appalachian  ridges  shows  ho\v  frequently  the  capture 
and  diversion  of  smaller  streams  by  larger  ones  has  been  accom- 
plished among  those  mountains. 

Figures  134  and  135  show  two  stages  in  the  evolution  of  a  river 
system.  Figure  134  represents  the  first  stage,  in  which  several 
transverse  streams,  a,  c,  e,f,  g,  are  breaching  the  escarpments  indi- 
cated by  shaded  lines.  Of  these  streams  c  carries  the  most  water, 
and  will  therefore  deepen  its  gorges  through  the  hard  ridges  more 
rapidly  than  the  others,  and  give  its  tributaries  the  advantage  of 
a  greater  fall.  In  the  second  stage  (Fig.  135),  c  has  captured  the 
upper  courses  of  all  the  other  streams  except  g,  which  has  not  yet 
been  reached.  The  branch  /  has  captured  a,  beheading  it,  divert- 


MATURE   DRAINAGE   SYSTEMS 


329 


ing  the  portion  a"  and  reversing  the  portion  a'.  Similarly,  ;;/  has 
captured  and  divided  e,  n  has  done  the  same  with  b,  and/  with  d, 
while  g  must  eventually  suffer  the  same  fate.  Wind  gaps  will  be 
left  in  the  ridges  where  the  captured  streams  once  crossed  them. 

In  regions  of  folded  rocks  thrown  into  a  series  of  parallel  anticlines 
and  synclines,  the  process  of  adjustment  may  become  exceedingly 
complicated.  Suppose  an  original  consequent  stream  flowing  in 
a  syncline  of  hard  rock  considerably  above  base-level,  whose  sub- 
sequent branches  have  opened  out  valleys  in  softer  rocks  along  the 


FIG.  134.  —  Evolution  of  a  river  system, 
first  stage.  The  shaded  lines  represent 
escarpments  of  hard  rock.  (De  Lap- 
parent.) 


FIG.  135.  —  Evolution  of  a  river 
system,  second  stage.  (De  Lap- 
parent.) 


crests  of  the  anticlines,  where  the  harder  surface  stratum  is  first  cut 
through.  The  extension  and  junction  of  these  subsequent  branches 
may  offer  a  more  advantageous  course  than  the  hard  syncline,  and 
cause  the  latter  to  be  wholly  or  partially  deserted.  The  streams 
originally  flowing  in  the  synclinal  troughs  may  gradually  be  shifted 
to  the  degraded  anticlines  which,  as  we  have  seen,  are  wasted 
away  more  rapidly. 

A  thoroughly  mature  drainage  system  is  characterized  by  a  com- 
plete adjustment  of  its  streams  to  the  structure  of  the  rocks.  The 
rivers  as  finally  established  are  thus  apt  to  be  a  patchwork  of  streams 
captured  and  diverted,  and  the  result  of  adjustment  is  the  produc- 


330  ADJUSTMENT   OF   RIVERS 

tion  of  a  system  often  radically  different  from  the  original  one. 
Even  after  a  river  system  has  become  maturely  adjusted,  a  reeleva- 
tion  of  the  country  may  produce  a  new  and  entirely  different  adjust- 
ment, by  changing  the  relation  of  the  folds  and  outcrops  of  hard 
and  soft  strata  to  the  base-level.  A  region  of  great  antiquity  which 
has  repeatedly  been  worn  down  and  reelevated  will  have  experienced 
many  revolutions  of  its  drainage  systems.  In  the  difficult  work  of 
deciphering  these  complex  histories  all  the  indications  of  abandoned 
stream  channels  must  be  carefully  examined.  Some  of  these,  like 
gravel  deposits  or  wind  gaps,  are  easy  to  find  and  to  trace  out,  but 
subsequent  denudation  will  frequently  have  removed  the  gravels 
and  otherwise  masked  the  old  stream  courses.  In  rocky  regions 
a  welcome  indication  of  ancient  stream  courses  is  often  given  by  pot 
holes,  which  are  deep,  circular,  well-like  openings  excavated  in  the 
rock,  and  wherever  they  occur  was  once  the  bed  of  a  stream.  A 
pot  hole  is  made  by  the  gyration  of  stones  which  are  whirled  around 
by  an  eddy  in  the  current  or  at  the  foot  of  a  waterfall  The  con- 
ditions must  remain  constant  for  some  time  for  the  hole  to  be  cut 
to  any  depth,  some  of  them  being  as  much  as  twenty  feet  deep. 

Accidents  to  Rivers.  —  This  term  is  employed  to  express  the 
interruptions  which  hinder  or  prevent  the  normal  development  of 
a  river  system.  The  diastrophic  changes  and  their  effects  we  have 
already  considered,  but  there  are  others  which  should  be  men- 
tioned. A  change  of  climate  from  moist  to  arid  greatly  interferes 
with  the  development  and  adjustment  of  a  river  system.  Many 
stream  channels  are  abandoned  and  others  are  occupied  only  after 
rains,  while  the  reduced  flow  in  the  permanent  streams  diminishes 
their  erosive  powers.  Large  areas,  like  the  Great  Basin  region, 
may  have  no  outlet  to  the  sea,  because  the  mountain  streams  all 
lose  themselves  in  the  desert  sands.  Lake  Bonneville  (see  p.  146) 
had  an  outlet  until  the  increasing  dryness  of  the  climate  so  lowered 
its  waters  that  the  outlet  could  no  longer  be  reached,  evaporation 
exceeding  influx.  Great  lava  flows  may  obliterate  the  drainage 
system  of  a  region  and  compel  the  establishment  of  an  entirely 
new  one,  as  has  happened  in  southern  Idaho  and  southeastern 
Oregon,  a  region  of  exceedingly  immature  topography  and  drain- 


ACCIDENTS  TO   RIVERS  331 

age.  Extensive  ice-sheets,  by  spreading  a  thick  mantle  of  drift 
which  fills  up  the  valleys,  may  produce  the  same  effects  as  lava 
flows,  except  that  the  drift  is  more  easily  removed.  In  the  north- 
eastern United  States  many  streams  have  been  displaced  by  the 
sheets  of  glacial  drift,  and  forced  to  seek  new  channels  at  a  com- 
paratively recent  date ;  they  still  preserve  all  the  signs  of  youth, 
such  as  deep,  trench-like  gorges  (see  Fig.  32),  waterfalls,  and 
rapids.  The  larger  rivers  have,  for  the  most  part,  been  able  to 
reoccupy  their  old  valleys,  but  the  smaller  streams  have  generally 
been  compelled  to  excavate  new  channels. 


CHAPTER   XIX 
MOUNTAIN  RANGES  —  CYCLES   OF  EROSION 

THE  term  mountain  is  somewhat  loosely  employed  for  any  lofty 
eminence,  and  the  distinction  between  mountains  and  hills,  as 
ordinarily  made,  is  principally  a  question  of  mere  height.  Some 
so-called  mountain  peaks  and  ridges  are  merely  the  portions  of 
dissected  plateaus  left  standing,  such  as  Lookout  Mountain  and 
Missionary  Ridge  in  Tennessee,  and  the  Allegheny  Front  in  Penn- 
sylvania. Such  mountains  usually  have  flat  tops  (table  mountains), 
are  composed  of  strata  which  are  nearly  or  quite  horizontal,  and 
owe  their  existence  either  to  their  being  composed  of  more  re- 
sistant rocks  than  the  denuded  parts  of  the  plateau,  or  to  their 
favourable  situation  with  reference  to  the  drainage  lines.  Another 
type  of  mountain  is  the  volcanic,  which  is  usually  an  isolated  cone 
and  may  be  built  up  to  great  heights  ;  it  is  simply  the  accumula- 
tion of  volcanic  material  which  has  been  piled  up  around  the 
vent.  Typical  mountain  ranges  and  chains  differ  materially  from 
either  of  these  classes,  both  in  their  structure  and  their  mode  of 
origin.  Before  proceeding  to  discuss  the  origin  and  history  of 
mountains,  it  will  be  necessary  to  define  the  terms  to  be  used. 

A  Mountain  Range  is  made  up  of  a  series  of  more  or  less  par- 
allel ridges,  all  of  which  were  formed  within  a  single  geosyncline 
(p.  236)  or  on  its  borders.  The  ridges  are  separated  from  one 
another  by  longitudinal  valleys  and  may  be  formed  either  by  the 
successive  folds  or  by  denudation  within  the  limits  of  the  folds. 
In  the  latter  case  the  outcropping  harder  strata  make  the  ridges. 
A  mountain  range  is  always  very  long  in  proportion  to  its  width, 
and  its  ridges  have  a  persistent  trend.  These  features  distinguish 
a  true  range  from  the  ridges  cut  out  of  a  plateau  by  denudation. 
The  Appalachian  range,  the  Wasatch,  the  Coast  Range,  are  ex- 
amples of  typical  mountain  ranges. 

332 


THICKNESS   OF   STRATA  333 

A  Mountain  System  is  made  up  of  a  number  of  parallel  or 
consecutive  ranges,  formed  in  separate  geosynclines,  but  of  ap- 
proximately similar  dates  of  upheaval.  The  Appalachian  system 
comprises  the  Appalachian  range,  running  from  New  York  to 
Georgia,  the  Acadian  range  in  Nova  Scotia  and  New  Brunswick, 
and  the  Otiachita  range  in  Arkansas.  Each  of  these  ranges  was 
formed  in  a  different  geosynclinal,  but  at  the  same  geological 
date,  and  they  are  consecutive,  having  a  common  direction. 

A  Mountain  Chain  comprises  two  or  more  systems  in  the  same 
general  region  of  elevation,  but  of  different  dates  of  origin.  The 
Appalachian  chain  includes  the  Appalachian  system,  the  Blue 
Ridge,  the  Highlands  of  New  Jersey  and  the  Hudson,  a  system  of 
different  date,  and  the  Taconic  system  of  western  New  England, 
which  was  not  formed  at  the  same  time  as  either  of  the  others. 

A  Cordillera  consists  of  several  mountain  chains  in  the  same 
part  of  the  continent.  Thus,  the  chains  of  the  Rocky  Mountains, 
Sierra  Nevada,  Coast  Range,  and  their  prolongations  in  Canada, 
together  make  up  the  Rocky  Mountain  or  Western  Cordillera. 

From  these  definitions  it  will  appear  that  the  mountain  range 
has  a  unity  of  structure  and  origin  which  fits  it  especially  for 
study.  If  the  structure  and  history.of  the  ranges  be  understood, 
the  systems  and  chains  will  offer  little  additional  difficulty. 

A  mountain  range  (disregarding,  for  the  present,  certain  excep- 
tional cases)  consists  of  a  very  thick  mass  of  strata,  which  are 
much  thicker  in  the  mountains  than  the  same  strata  in  the 
adjoining  plains.  In  the  Appalachian  range,  for  example,  the 
stratified  rocks  are  nearly  40,000  feet  thick,  but  on  tracing 
the  same  series  of  beds  westward  into  the  Mississippi  valley; 
they  are  found  to  become  very  much  thinner,  hardly  exceeding 
one-tenth  of  the  thickness  in  the  mountains.  This  immense  thick- 
ness of  the  component  strata  is  not  peculiar  to  the  Appalachians 
but  reappears  in  the  typical  mountain  ranges  everywhere  :  the 
VVasatch  range  has  31,000  feet  of  strata,  the  Coast  Range  30,000 
feet,  the  Alps  50,000  feet,  etc.  The  thick  series  of  strata  which 
make  up  a  mountain  chain  are  usually  conformable  throughout ; 
deposition  was,  for  the  most  part,  continuous,  and  there  was 


334  MOUNTAIN   RANGES 

little  or  no  loss  from  denudation,  though  in  some  cases  the 
region  which  subsequently  was  upheaved  into  the  range  had  its 
oscillations  of  level,  recorded  now  in  unconformities.  This  may 
be  seen,  for  example,  in  the  Ouachita  range  of  Arkansas. 

Another  well-nigh  universal  fact  concerning  the  structure  of 
mountain  ranges  is  the  intense  folding  or  plication  of  their  strata, 
often  accompanied  by  great  thrust  faults.  The  degree  of  plica- 
tion varies  much  in  different  ranges.  The  Uinta  Mountains  are 
formed  by  a  single  great  and  gently  swelling  arch  of  strata,  faulted 
along  its  northern  slope.  So  gentle  is  the  curvature-  of  the  beds 
that  in  a  single  view  they  often  seem  to  be  quite  horizontal.  Much 
more  commonly  the  strata  are  thrown  into  a  series  of  parallel 
folds,  which  sometimes  are  open,  upright,  and  symmetrical,  as  in 
the  Jura  Mountains  of  Switzerland ;  its  folds  are  so  symmetrical 
and  regular  that  a  section  across  the  parallel  ridges  looks  like  a 
diagram.  This  comparatively  gentle  folding  is,  however,  not  the 
rule,  but  rather  an  intense  compression  and  plication.  The  Appa- 
lachians are  thrown  into  closed,  asymmetrical,  and  overturned  folds, 
with  frequent  great  thrust  faults  (see  Fig.  93,  p.  238).  The  Sierra 
Nevada  is  so  intensely  plicated  that  the  thickness  of  its  strata  has 
not  yet  been  estimated.  The  Alps  have  undergone  such  enormous 
compression  that  many  of  the  ridges  are  in  the  form  of  fan  folds 
(i.e.  the  anticlines  are  broader  at  the  crest  than  at  the  base), 
while  others  have  been  pushed  over  to  an  inverted  position. 
The  combination  of  this  violent  contortion  with  faults  and  thrusts 
often  results  in  an  indescribable  confusion  and  chaos  of  forms, 
which  it  is  exceedingly  difficult  to  comprehend. 

The  two  main  characteristic  features  of  mountain  ranges  are, 
then,  the  immense  thickness  of  the  strata  of  which  they  are  made, 
and  the  compression  and  folding  or  faulting  which  they  have  un- 
dergone. Certain  minor  structures  which  accompany  these  more 
striking  features  should,  however,  not  be  overlooked.  In  the  first 
place,  the  folded  strata  of  mountain  ranges  are  very  generally 
cleaved,  or  fissile,  or  both,  the  planes  of  cleavage  or  fissility 
running  parallel  with  the  axes  of  the  folds.  (2)  The  major 
folds  are  themselves  composed  of  successive  series  of  minor  folds 


ORIGIN  OF   RANGES  335 

in  descending  order  of  magnitude,  the  smallest  of  them  being 
visible  only  with  the  microscope.  (3)  Dynamic  metamorphism 
is  an  almost  universal  feature  of  mountain  ranges,  the  transforma- 
tion of  the  rocks  being  in  proportion  to  the  intensity  of  the  plica- 
tion. The  microscope  gives  eloquent  testimony  to  the  enormous 
forces  which  have  been  at  work,  by  showing  how  the  minerals  have 
been  mashed  and  flattened,  rendered  plastic  and  flowing  like  wax 
in  a  hydraulic  press.  (4)  Masses  of  igneous  rocks  are  very  often, 
though  not  always,  associated  with  mountain  ranges,  and  many 
such  ranges  have  a  core  of  igneous  rock,  often  granite,  with 
strata  flanking  it  on  both  sides. 

ORIGIN  OF  MOUNTAIN  RANGES 

The  manner  in  which  mountain  ranges  have  been  formed  must 
be  deduced  from  a  careful  study  of  their  structure,  for  no  one 
has  ever  witnessed  the  process  of  that  formation.  Mountain 
building  may  be  going  on  at  the  present  time ;  indeed,  there 
is  no  reason  to  suppose  that  it  is  not,  but  so  slowly  is  the  work 
carried  on  that  it  withdraws  itself  entirely  from  observation. 
Nevertheless  the  general  course  of  events  may  be  inferred  with 
much  confidence  from  the  structure  of  the  range. 

The  first  step  in  the  formation  of  a  mountain  range  must  evi- 
dently be  the  accumulation  of  an  immensely  thick  body  of  strata. 
This,  of  course,  must  have  taken  place  under  water,  and  the  only 
body  of  water  large  enough  is  the  sea.  Furthermore,  our  studies 
of  modern  marine  deposits  have  taught  us  that  thick  strata  can  be 
accumulated  only  in  rather  shallow  water  and  parallel  with  shore- 
lines. This  shoal  water  origin  of  their  strata  is  confirmed  by  the 
examination  of  actual  mountain  ranges,  where  we  find  great  masses 
of  conglomerates,  ripple-marked  and  sun- cracked  sandstones  and 
shales,  and  abundant  other  testimony  of  deposition  in  shallow 
water.  To  accumulate  thick  strata  in  shoal  water  the  bottom 
must  subside  as  the  sediments  are  piled  upon  it,  else  the  water 
would  be  filled  up  and  deposition  cease.  Such  a  sinking  trough 
is  a  geosyncline,  and  in  geosynclines  filled  with  sediments  is  the 


336  MOUNTAIN   RANGES 

cradle  of  the  mountains.  The  area  of  the  trough  varies  from 
time  to  time,  as  do  also  the  position  of  the  line  of  maximum 
subsidence  and  the  relative  rate  of  depression  and  sedimenta- 
tion, so  that  the  depth  of  water  varies.  We  saw  above  that 
the  strata  of  mountain  ranges  are  very  much  thicker  than  the 
same  strata  in  the  adjoining  plains,  which  means  that  the  ranges 
have  been  formed  along  the  lines  of  maximum  sedimentation. 

The  second  stage  in  the  building  of  a  range  is  the  upheaval  of 
the  thick  mass  of  strata  into  a  series  of  anticlinal  and  synclinal 
folds,  which  may  be  upright,  open,  and  symmetrical,  or  closed, 
asymmetrical,  inclined,  or  inverted.  This,  as  we  have  already 
learned,  can  be  produced  only  by  lateral  compression,  a  con- 
clusion which  is  sustained  not  only  by  the  mechanics  of  fold- 
ing and  faulting,  but  also  by  the  less  obvious  structures,  such 
as  cleavage  and  fissility,  metamorphism,  the  microscopic  crum- 
plings  and  plications,  and  the  crushing  and  flowage  of  the  mineral 
particles.  Experiments  upon  the  lateral  compression  of  plastic 
substances  under  load  give  the  same  result.  The  compress- 
ing force  does  not  raise  anticlines  with  great  cavities  beneath 
them,  for  such  arches  could  not  well  be  self-supporting,  but 
mashes  together  the  whole  mass  of  strata,  raising  them  into  folds 
and  wrinkles,  crowding  the  beds  into  a  greatly  reduced  breadth ; 
or  when  they  are  not  sufficiently  loaded  to  be  plastic,  break- 
ing and  dislocating  them  in  great  thrust  faults.  Certain  mountain 
areas  in  Pennsylvania  have  been  compressed  into  one-sixth  the 
breadth  originally  occupied.  It  is  not  necessary  to  suppose  that 
a  mountain  range  was  thrown  up  by  one  steady  movement.  On 
the  contrary,  there  is  good  reason  to  believe  that  repeated  move- 
ments, separated  it  may  be  by  long  intervals  of  time,  have  been 
engaged  in  the  work. 

That  mountain  ranges  have  been  forced  upward  by  lateral  com- 
pression is  an  unquestioned  fact,  but  to  determine  how  that  com- 
pression was  generated  is  a  much  more  difficult  problem.  The 
most  satisfactory  explanation  yet  offered  is  that  the  compression 
is  due  to  the  contraction  of  the  globe  from  cooling.  The  earth's 
crust  long  ago  reached  a  state  of  fairly  constant  temperature,  but 


BASIN   RANGES 


337 


the  highly  heated  interior  is  steadily  cooling  by  radiation,  and  con- 
tracting. The  crust  must  follow  the  shrinking  interior,  and  is 
thereby  crowded  into  a  smaller  space,  which  sets  up  irresistible 
lateral  pressure,  to  which  the  crust  must  give  way,  even  though  it 
were  far  more  rigid  than  steel.  A  withered  apple  has  a  wrinkled 
skin  because  the  fruit  has  shrunk  from  loss  of  water,  and  the  skin, 
crowded  into  a  smaller  space,  is  folded  and  wrinkled. 

Various  objections  have  been  urged  against  the  contraction 
theory,  chiefly  on  the  ground  that  the  cause  is  insufficient  to  do 
the  work  demanded  of  it.  Those  objections  cannot  be  com- 
pletely answered  because  of  our  ignorance  of  the  quantitative 
factors  involved  in  the  problem,  but  the  fact  remains  that  no 
other  suggestion  explains  the  facts  of  mountain  structure  so  well. 


FlG.    136.  —  The   Charleston    Mountains,    Nevada.      One   of  the   Basin   Ranges. 
(Photograph  by  Merriam.) 

There  are  certain  mountain  ranges  which  have  an  exceptional 
structure  and  must  have  had  a  correspondingly  different  mode  of 
origin.  In  the  Great  Basin  which  lies  between  the  Sierra  Nevada 
and  the  Wasatch  Mountains,  are  a  number  of  parallel  mountain 
ranges  with  a  prevalent  north  and  south  trend,  which  are  collec- 
tively called  the  Basin  Ranges.  These  mountains  are  not  folds 
of  very  thick  strata,  but  tilted  fault  blocks,  which  have  been 
made  by  normal  faults,  each  upthrow  side  standing  as  a  great 


338  MOUNTAIN   RANGES 

escarpment,  but  with  a  tilted  top  that  gradually  slopes  back  to  the 
foot  of  the  next  block,  to  which  it  stands  as  the  downthrow  side. 
The  processes  of  denudation  have  carved  these  tilted  blocks  into 
peaks  and  ridges  of  the  ordinary  kind.  The  boundary  ranges, 
the  Sierra  Nevada  and  the  Wasatch,  although  mountains  of  fold- 
ing, have  themselves  been  modified  by  the  same  process,  for 
each  of  these  ranges  has  a  great  fault  along  its  base,  the  Great 
Basin  being  on  the  downthrow  side  with  reference  to  each  of 
them.  The  fault  of  the  Sierras  is  on  the  east  side  of  the  range  and 
hades  toward  the  east,  that  of  the  Wasatch  is  on  the  west  side  and 
hades  to  the  west.  The  dislocations  of  this  region  have  been  kept 
up  till  a  very  recent  period ;  in  southeastern  Oregon  the  fault 
scarps  are  still  very  plainly  shown,  the  upthrow  sides  forming 
ridges,  and  the  downthrow  sides  valleys  in  many  of  which  water 
has  gathered  into  lakes  (see  Fig.  104).  Near  Salt  Lake  City  the 
movements  may  even  yet  be  in  progress.  In  Fig.  52  (p.  132)  is 
shown  an  alluvial  cone  at  the  foot  of  the  Wasatch  Mountains ;  on 
the  right  side  of  the  cone,  near  its  upper  end,  may  be  seen  a  low 
fault  scarp,  which  could  not  have  been  very  long  maintained  in 
such  incoherent  materials.  This  exceptional  mountain  structure 
seen  in  the  Basin  Ranges  is,  then,  due  to  normal  faulting. 

Another  type  of  mountain  structure  different  from  either  of 
those  mentioned  is  the  laccolithic  mountain.  A  laccolith  (see 
p.  283)  is  a  rounded,  intrusive  mass  of  igneous  rock  which  has 
lifted  up  an  arch  of  strata  above  it  into  a  dome,  but  has  not  reached 
the  surface  to  flow  out  as  lava.  A  laccolithic  mountain  may  stand 
isolated,  like  Little  Sun  Dance  Hill  (see  Fig.  128),  or  several  of 
them  may  be  grouped  together,  as  in  the  Henry  Mountains  of 
southern  Utah,  or  again,  they  may  form  extensive  portions  of  true 
ranges,  as  in  the  Elk  Mountains  of  Colorado. 

The  Date  of  Mountain  Ranges  means  the  geological  period  in 
which  they  were  first  upheaved  above  the  sea.  This  date  is  sub- 
sequent to  the  newest  strata  which  are  involved  in  the  movement, 
and  earlier  than  that  of  the  oldest  strata  which  did  not  take  part 
in  the  movement,  but  must  have  done  so,  had  they  been  present. 
Strata  which  rest  unconformably  against  the  flanks  of  a  range  must 


DENUDATION  339 

have  been  deposited  after  the  folding  movement  was  accom- 
plished. If  the  newest  folded  strata  and  the  oldest  unmoved 
strata  be  of  successive  geological  periods,  the  date  of  the  upheaval 
is  placed  between  those  two  periods  and  said  to  close  the  older 
one  for  the  particular  region  involved.  The  subsequent  history 
of  a  mountain  range  after  its  final  upheaval  above  the  sea  must  be 
read  in  its  denudation  and  in  the  evolution  of  its  topography  and 


drainage. 


DENUDATION  OF  MOUNTAINS 


Mountains  as  we  see  them  are  never  in  the  shape  which  they 
would  present,  were  the  forces  of  compression  and  upheaval  alone 
concerned  in  their  formation.  Every  mountain  range  has  been 
profoundly  affected  by  the  agencies  of  denudation,  and  their 
ridges  and  peaks,  their  cliffs  and  valleys,  have  been  carved  out  of 
swelling  folds  and  domes,  or  angular,  tilted  fault  blocks.  As 
upheaval  is  a  slow  process,  denudation  must  have  begun  its  work 
as  soon  as  the  crests  of  the  folds  made  their  appearance  above 
the  sea,  or  above  the  level  of  the  ground,  so  that  probably  no 
range  ever  had  the  full  height  which  the  strata,  if  free  from  de- 
nudation, would  have  given  to  it.  Upheaval,  though  sometimes 
slow  enough  to  allow  rivers  to  keep  their  channels  open,  is  yet  too 
rapid  to  be  kept  in  check  by  the  processes  of  general  atmospheric 
weathering,  and  so  the  ranges  grew  into  great  uplifts.  But  as  soon 
as  the  movement  of  elevation  ceased,  denudation  began  to  get  the 
upper  hand,  for  as  we  have  learned,  mountains  are  the  scene  of 
exceptionally  rapid  erosion.  The  steepness  of  their  sides  gives 
great  power  to  the  streams  which  course  down  them,  they  cause 
the  discharge  of  the  atmospheric  moisture  in  rain  or  snow,  they 
are  terribly  riven  by  the  frost,  and  they  are  frequently  cut  and 
gashed  by  glaciers.  For  a  long  period  the  effect  of  denudation 
is  to  greatly  increase  the  ruggedness  of  the  mountains,  carving 
folds  into  ridges  and  cliffs,  and  ridges  into  bold  and  inaccessible 
peaks,  but  sooner  or  later  the  mountains  are  worn  down  lower 
and  lower,  and  are  eventually  levelled  with  the  plains  from  which 
they  spring.  In  the  process  of  degradation,  the  synclines  often 


340  CYCLES   OF   DENUDATION 

resist  wear  better  than  the  anticlines,  and  standing  up  above  the 
level,  form  the  synclinal  mountains  of  many  ancient  ranges. 

From  the  geological  point  of  view  mountains  must  be  regarded 
as  short-lived  and  ephemeral ;  low-lying  plains  persist  for  a  longer 
time  than  do  lofty  ranges,  as  rivers  may  outlast  many  generations 
of  lakes.  Consequently,  among  the  mountain  chains  of  the  globe, 
we  everywhere  find  that  the  lofty  ranges  are  those  of  compara- 
tively recent  date,  while  ancient  mountains  have  been  worn  down 
into  mere  stumps.  The  Appalachians  have  been  reduced  nearly 
to  base-level,  and  their  present  condition  is  that  of  a  reelevated 
and  dissected  peneplain,  the  ridges  and  valleys  of  which  are  deter- 
mined by  the  position,  attitude,  and  alternation  of  harder  and 
softer  strata.  In  its  pristine  state  this  very  ancient  range  may 
well  have  been  as  lofty  as  the  Alps  or  Andes.  Of  course,  there 
is  no  mathematical  ratio  between  the  youth  of  a  range  and  its 
height,  for  moderately  folded  strata  of  moderate  thickness  never 
could  have  formed  very  high  mountains,  but  in  a  general  way  it  is 
true,  that  very  high  ranges  are  youthful,  and  that  very  old  ranges 
are  low.  The  process  of  degradation  may  go  so  far  as  to  sweep 
away  a  mountain  range  to  its  very  roots,  leaving  only  the  intensely 
plicated  strata  of  the  plain  as  evidence  that  mountains  ever 
existed  there.  Of  such  a  nature  is  the  upland  of  southern  New 
England  and  the  great  metamorphic  area  of  Canada,  both  of 
which  probably  once  carried  ranges  of  high  mountains. 

SUCCESSIVE  CYCLES  OF  DENUDATION. 

We  have  seen  that  any  region,  however  lofty  and  rugged,  must 
eventually  be  worn  down  to  base-level,  provided  only  that  the 
country  remain  stationary  with  reference  to  the  sea  until  the  pro- 
cess of  degradation  is  complete.  It  is  doubtful,  however,  whether 
any  extensive  region  of  hard  rocks  has  ever  been  absolutely  reduced 
to  base-level :  the  usual  result  is  the  formation  of  a  peneplain,  a 
low-lying,  featureless  surface  of  gentle  slopes  and  with  only  occa- 
sional eminences  rising  above  the  general  level.  Reelevation  of 
such  a  peneplain  at  once  revivifies  the  streams  and  gives  all  the 


APPALACHIAN    CYCLES  34! 

destructive  agencies  new  powers.  The  peneplain  is  attacked  and 
carved  into  valleys  and  hills,  the  valleys  being  rapidly  cut  down 
to  base-level,  while  the  divides  and  hills  are  much  more  slowly 
removed.  If  time  enough  be  granted,  the  rugged  country  formed 
from  a  dissected  peneplain  is  in  its  turn  worn  down  to  a  second 
peneplain  at  a  lower  base-level.  This  alternate  upheaval  and 
wearing  down  together  constitute  a  cycle  of  denudation,  from  base- 
level  back  to  base-level.  A  complete  cycle  is  one  in  which  the 
whole  region  is  reduced  to  a  peneplain  before  the  reelevation 
occurs,  and  a  partial  or  incomplete  cycle  is  one  which  is  inter- 
rupted by  upheaval  before  the  region  is  cut  down,  and  only  small 
and  local  peneplains  have  been  formed.  From  a  study  of  an  old 
region  several  cycles  of  denudation  may  frequently  be  made  out, 
represented  by  the  remnants  of  dissected  peneplains  at  different 
levels  preserved  in  the  harder  rocks.  The  successive  adjustments 
of  the  drainage  system  are  a  valuable  auxiliary  in  working  out  the 
history  of  the  cycles. 

As  an  excellent  example  of  these  cycles  of  denudation  whose 
marks  are  preserved  in  the  structure  of  the  rocks,  we  may  take 
the  Appalachian  Mountains,  which  have  been  studied  with  great 
care.  The  cycles  have  been  worked  out  elaborately,  but  only  an 
outline  of  the  more  striking  events  can  be  given  here. 

These  mountains  began  as  a  great  geosyncline  in  which 
throughout  the  vast  lengths  of  the  Palaeozoic  era  (see  p.  365) 
were  accumulated  enormously  thick  masses  of  shoal  water  sedi- 
ments. At  the  close  of  that  era  a  number  of  crustal  movements 
set  in,  crushing  the  sides  of  the  geosynclinal  trough,  and  crum- 
pling the  mass  of  strata  contained  in  it  into  a  series  of  roughly 
parallel,  closed,  inclined,  or  overturned  folds,  forming  doubtless 
a  very  lofty  range  of  mountains.  During  the  long  ages  of  the 
Mesozoic  era  (see  p.  441)  the  mountains  were  attacked  and  worn 
down  by  the  destructive  agencies ;  and  by  the  time  the  Creta- 
ceous period  was  reached  (see  p.  474)  the  range  had  been  re- 
duced to  a  peneplain,  with  only  a  few  hills  rising  above  its 
almost  featureless  level,  —  hills  which  are  now  the  peaks  of  west- 
ern North  Carolina,  the  highest  points  of  the  range  at  present. 


342  CYCLES   OF   DENUDATION 

The  present  height  of  these  peaks  is  due  to  subsequent  reelevation. 
This  plain  is  called  the  Kittatinny  peneplain,  because  the  ridge 
of  that  name  in  Pennsylvania  and  New  Jersey  is  one  of  the  rem- 
nants of  it.  To  the  observer  who  can  overlook  the  billowy  ridges 
of  the  present  range  their  even  sky  line  is  very  striking,  and  these 
ridges  are  all  composed  of  the  hardest  rocks,  which  all  rise  to 
nearly  the  same  level.  To  reproduce  the  plain  it  would  be 
necessary  to  fill  the  valleys  between  the  Blue  Ridge  on  the  east 
and  the  plateau  on  the  west  up  to  the  level  attained  by  the  hard 
ridges,  and  this  would  give  a  gently  arched  surface,  sloping  very 
gradually  to  the  Mississippi  valley  and  the  Atlantic.  On  this 
peneplain  were  already  established  the  great  streams  which  flow 
to  the  ocean,  such  as  the  Susquehanna  and  the  Potomac. 

Next  the  peneplain  was  raised  very  gradually  to  a  height  of 
1400  feet  in  Virginia,  diminishing  in  both  directions  from  this 
point,  and  the  denuding  forces  once  more  attacked  and  dissected 
the  plain,  the  larger  streams  holding  their  transverse  courses  and 
sawing  through  the  hard  strata,  which  were  left  standing  as  ridges 
by  the  cutting  of  the  longitudinal  valleys  along  the  more  destructi- 
ble beds.  Denudation  had  cut  down  the  softer  beds  to  one  gen- 
eral level,  called  the  Shenandoah  peneplain,  the  period  of  rest 
being  long  enough  to  bring  all  the  areas  of  soft  and  soluble  beds 
to  this  level,  but  not  to  materially  lower  the  ridges  of  the  more 
resistant  strata. 

"  The  swelling  of  the  Appalachian  dome  began  again.  It  rose 
200  feet  in  New  Jersey,  600  feet  in  Pennsylvania,  1700  feet  in 
southern  Virginia,  and  thence  southward  sloped  to  the  Gulf  of 
Mexico.  ...  In  consequence  of  the  renewed  elevation,  the 
streams  were  revived.  Once  more  falling  swiftly,  they  have 
sawed,  and  are  sawing,  their  channels  down,  and  are  preparing 
for  the  development  of  a  future  base-level."  (Willis.) 

This  example  is  sufficient  to  show  the  value  of  the  physio- 
graphic method  to  the  geologist  in  supplementing  the  knowledge 
derived  from  the  study  of  sedimentation. 


PART   IV 

HISTORICAL   GEOLOGY 

CHAPTER  XX 
FOSSILS 

A  FOSSIL  is  the  impression  or  remains  of  an  animal  or  plant 
which  has  been  entombed  in  the  rocks  by  natural  causes. 

A  knowledge  of  fossils  is  indispensable  to  the  geologist  because 
they  give  him  the  means  of  establishing  a  consecutive  chronology 
of  the  earth,  and  teach  him  much  concerning  the  changes  of 
land  and  sea,  of  climate,  and  of  the  distribution  of  living  things 
upon  the  globe.  To  comprehend  the  lessons  taught  by  fossils,  it 
is  essential  not  only  that  the  student  should  familiarize  himself 
with  actual  specimens,  but  also  that  he  should  have  some  ac- 
quaintance with  the  elements  of  zoology  and  botany,  else  he  can- 
not appreciate  the  distinctions  which  obtain  between  the  fossils  of 
widely  separated  periods  of  time. 

I.     HOW   FOSSILS  WERE    EMBEDDED   IN  THE   ROCKS, 

The  conditions  of  the  preservation  of  fossils  are  much  more 
favourable  to  some  kinds  of  organisms  than  to  others.  It  is  only 
tinder  the  rarest  circumstances  that  soft,  gelatinous  animals,  which 
(like  jelly-fish)  have  no  hard  parts,  can  leave  traces  in  the  rocks. 
The  vast  majority  of  fossilized  animals  are  those  which  have  hard 
shells,  scales,  teeth,  or  bones ;  and  of  plants,  those  which  contain 
a  sufficient  amount  of  woody  tissue. 

343 


344  FOSSILS 

Again,  the  conditions  under  which  organisms  live  have  a  great 
influence  upon  the  chances  of  their  preservation  as  fossils.  Land 
animals  and  plants  are  much  less  favourably  situated  than  are 
aquatic  forms,  and  since  by  far  the  greater  number  of  sedimentary 
rocks  were  laid  down  in  the  sea,  marine  organisms  are  much  more 
common  as  fossils  than  are  those  of  fresh  water. 

Few  rocks,  and  those  unimportant,  are  accumulated  on  land, 
and  so  fossils  are  rarely  preserved  on  land  surfaces,  though  they  are 
occasionally  buried  in  blown  sand  or  in  the  soil.  Peat  bogs  are, 
however,  excellent  places  for  fossilization,  and  the  coal  seams 
have  yielded  great  numbers  of  fossils,  principally  of  plants.  The 
remains  of  land  animals  and  plants,  especially  of  the  latter,  are 
sometimes  swept  out  to  sea,  sink  to  the  bottom,  and  are  there 
covered  up  and  preserved  in  the  deposits ;  but  such  occurrences 
are  relatively  uncommon.  Lakes  offer  much  more  favourable 
conditions  for  the  preservation  of  terrestrial  organisms.  Sur- 
rounding trees  drop  their  leaves,  flowers,  and  fruit  upon  the  flats 
and  shallows  of  fine  mud,  insects  fall  into  the  quiet  waters, 
while  quadrupeds  are  mired  in  mud  or  quicksand  and  soon 
buried  out  of  sight.  Flooded  streams  bring  in  quantities  of 
vegetable  de"bris,  together  with  the  carcases  of  land  animals, 
drowned  by  the  sudden  rise  of  the  flood.  When  the  carcase  of 
a  freshly  drowned  mammal  is  washed  into  the  lake,  it  immedi- 
diately  sinks  to  the  bottom  ;  and  if  sufficient  sediment  be  de- 
posited upon  it,  it  will  be  held  there  and  fossilized  as  a  complete 
skeleton.  But  if  little  sediment  be  thrown  down  upon  it,  the 
carcase  will  rise  and  float,  when  the  body  is  distended  by  the  gases 
of  decomposition.  Floating  thus,  it  will  be  pulled  about  by  the 
flesh-eating  creatures  which  inhabit  the  lake,  dropping  one  bone 
here  and  another  there,  over  a  wide  area. 

The  great  series  of  lake  deposits,  which  for  long  ages  were 
formed  in  various  parts  of  our  West,  have  proved  to  be  a  mar- 
vellous museum  of  the  land  and  fresh-water  life  of  that  region. 
On  the  fine-grained  shales  are  preserved  innumerable  insects  and 
fishes,  with  multitudes  of  leaves,  many  fruits  and  occasionally 
flowers,  while  in  the  sands  and  clays  are  entombed  the  bones  of 


MODE   OF  PRESERVATION  345 

the  reptiles,  mammals,  and,  more  rarely,  birds  of  the  land, 
mingled  with  those  of  the  crocodiles,  turtles,  and  fishes  that  lived 
in  the  water.  Similar  lake-beds  are  known  in  other  continents. 

It  is  on  the  sea-bed  that  the  conditions  are  most  favourable 
to  the  preservation  of  the  greatest  number  and  variety  of  fossils. 
Among  the  littoral  deposits  ground  by  the  ceaseless  action  of  the 
surf,  fossils  are  not  likely  to  be  abundant  or  well  preserved,  but 
in  quieter  and  deeper  waters  vast  numbers  of  dead  shells  and  the 
like  accumulate  and  are  buried  in  sediments.  The  fossils  are 
not,  however,  uniformly  distributed  over  the  sea-bottom  ;  in  some 
places  they  are  crowded  together  in  multitudes,  while  large  areas 
will  be  almost  devoid  of  them.  The  differences  are  due  to  variations 
in  temperature,  in  the  character  of  the  bottom,  in  food  supply, 
and  other  conditions.  In  tropical  seas,  swept  by  currents  which 
bring  in  abundant  supplies  of  food,  the  luxuriance  of  life  is  wonder- 
ful, and  in  such  places  a  correspondingly  large  number  of  fossils 
are  preserved.  However,  even  under  the  most  favourable  circum- 
stances, the  fossils  can  never  represent  more  than  a  fraction  of  the 
life  of  their  times.  Indeed,  the  wonder  is  that  so  much  of  the 
life  systems  of  past  ages  has  been  preserved,  rather  than  that  so 
large  a  part  lias  been  irretrievably  lost. 

The  ways  in  which  fossils  are  preserved  vary  much,  according  to 
circumstances,  but  three  groups  include  all  the  principal  kinds. 

(i)  Preservation  of  more  or  less  of  the  original  substance. 
In  certain  rare  instances  an  organism  may  be  preserved  intact, 
as  have  been  the  carcases  of  the  extinct  species  of  elephant  and 
rhinoceros  which  are  found  in  the  frozen  gravels  of  Siberia, 
which  after  thousands  of  years  of  burial  are  still  eagerly  devoured 
by  the  wolves.  Much  more  common  is  the  decomposition  of 
the  soft  structures  and  the  preservation  of  the  hard  parts, — 
bones,  shells,  etc.  Most  of  the  shells  and  bones  found  in  the 
rocks  of  later  geological  date  arc  composed  of  the  material  origi- 
nally belonging  to  them,  though  they  have  suffered  much  loss 
of  substance.  The  carbon  of  coal  plants  is  that  which  was 
present  in  the  living  vegetation,  but  the  volatile  matters  have 
disappeared. 


346  FOSSILS 

(2)  Entire  loss  of  substance  and  retention  of  form.     In  this 
class  of  fossils  all  the  original  material  of  the  organism  has  been 
lost,  and  no  trace  of  its  internal  structure  is  retained,  but  only  the 
external  form  has  been  reproduced  in  some   different   material. 
Under  this  class  we  may  distinguish  two  principal  varieties  :    (a) 
Moulds  and  (b)    Casts.     A  mould  is   formed  when   the  fossil   is 
embedded  in  sediments,  which  accurately  reproduce  its  external 
form,  and  harden  so  as  not  to  collapse  when  the  fossil  is  removed. 
Percolating  waters  then  dissolve  away  the  organism  entirely,  leav- 
ing only  a  cavity,  which  is  the  mould.     It   is  often  possible   to 
reproduce  the  form  of  a  vanished  fossil  by  filling  the  natural  mould 
with  plaster   of  Paris,  gutta-percha,  or   similar   substance.     Im- 
pressions of  footprints,  which  may  be  placed  in  the  same  category 
as  moulds,  have  already  been  explained  (see  p.  227). 

Casts  are  formed  when  the  mould  is  filled  by  some  solid  sub- 
stance deposited  from  percolating  waters,  thus  reproducing  the 
form  of  the  fossil,  as  is  done  artificially  with  plaster  or  gutta-percha. 
If  the  fossil  were  hollow,  like  a  shell,  we  frequently  find  a  com- 
bination of  internal  cast  with  an  external  mould  in  the  same 
specimen.  At  the  time  the  fossil  is  embedded  its  interior  is 
filled  with  the  same  sediment,  which  hardens  and  forms  an  inter- 
nal cast,  exactly  reproducing  the  form  of  the  interior.  The  shell 
itself  is  then  dissolved  away,  leaving  a  space  between  the  outer 
mould  and  the  inner  cast.  Moulds  and  casts  are  commonest  in 
rocks  which  permit  percolating  waters  to  traverse  them  freely, 
such  as  sandstones  and  coarse-grained  limestones.  An  interesting 
form  of  cast  is  the  brain-cast,  which  is  made  by  the  fine-grained 
sediment  which  fills  the  cranial  cavity  of  an  embedded  skull, 
often  reproducing  the  form  of  the  brain  with  much  accuracy. 

(3)  Loss  of  substance  with  reproduction  of  form  and  structure. 
Fossils  of  this  class  are  also  called  petrifactions  and  pseudomorphs 
(the  latter  a  term  borrowed  from  mineralogy).     Here  the  original 
material  of  the  organism  has  been  more  or  less  completely  removed, 
and  other  material  substituted  for  it ;  but  the  substitution  has  been 
so  gradual,  molecule  by  molecule,  that  not  only  the  external  form 
but  also  the  microscopic  structure  has  been  perfectly  reproduced. 


DETERMINING   CHRONOLOGY 


347 


Several  scantily  soluble  substances  act  as  petrifying  materials,  the 
most  perfect  results  being  given  by  silica.  A  silicified  bone,  or 
tooth,  or  bit  of  wood,  differs  from  the  original  only  in  weight, 
colour,  and  hardness,  and  when  a  thin  section  is  examined  under 
the  microscope,  the  minutest  details  of  structure  may  be  made  out 
as  perfectly  as  from  the  unaltered  original.  CaCO3  is  a  very 
common  petrifying  agent,  but  it  often  obliterates  structure  by 
crystallizing  after  deposition;  less  usual  are  iron  pyrites  and 

siderite.     It  need  scarcely  be  said  that  only  hard  substances 

wood,  bones,  shells,  etc.  —  are  sufficiently  durable  to  be  preserved 
in  this  way,  though  a  petrifaction  of  soft  tissues  has  been  reported 
as  occurring  under  very  exceptional  circumstances. 

II.   WHAT  MAY  BE  LEARNED  FROM  FOSSILS 

The  principal  value  which  fossils  possess  for  the  geologist  lies  in 
the  assistance  which  they  give  him  in  reconstructing  the  history  of 
the  globe.  This  they  do  in  several  ways. 

(i)  In  determining  Geological  Chronology.  —  The  most  obvious 
way  in  which  to  make  out  the  relative  ages  of  a  series  of  stratified 
rocks  is  to  determine  their  order  of  superposition,  for  the  oldest 
will  be  at  the  bottom  and  the  newest  at  the  top  (see  p.  221).  But 
this  method  is  of  only  local  application  and  will  not  carry  us  far  in 
an  endeavour  to  compile  a  history  of  the  whole  earth.  It  cannot 
enable  us  to  compare  even  the  rocks  of  different  parts  of  the  same 
continent,  for  any  exposed  section  is  but  a  small  fraction  of  the 
whole  series  of  strata.  More  embarrassing  still,  strata  change  their 
character  from  point  to  point,  limestone  being  laid  down  in  one 
place  while  sandstone  is  accumulating  in  another.  Still  less  can 
the  order  of  superposition  help  to  determine  the  relative  ages  of 
rocks  in  different  continents,  for  this  order  in  North  America  can 
be  no  guide  to  the  succession  in  Africa  or  Australia.  This  conclu- 
sion does  not  imply  that  order  of  superposition  may  be  safely 
neglected ;  on  the  contrary,  it  is  an  indispensable  aid,  but  it  must 
be  studied  in  connection  with  the  fossils. 

Since  the  first  introduction  of  life  on  the  globe  it  has  gone  on 


348  FOSSILS 

advancing,  diversifying,  and  continually  rising  to  higher  and  higher 
planes.  We  need  not  stop  to  inquire  how  this  progression  has 
been  effected ;  for  our  present  purpose  it  is  sufficient  to  know  that 
progress  and  change  have  been  unceasing  and  gradual,  though 
not  necessarily  occurring  at  a  uniform  rate.  Accepting,  then,  the 
undoubted  fact  of  the  universal  change  in  the  character  of  the 
organic  beings  which  have  successively  lived  on  the  earth,  it  follows 
that  rocks  which  have  been  formed  in  widely  separated  periods  of 
time  will  contain  markedly  different  fossils,  while  those  which  were 
laid  down  more  or  less  contemporaneously  will  have  similar  fossils. 
This  principle  enables  us  to  compare  and  correlate  rocks  from  all 
the  continents  and,  in  a  general  way,  to  arrange  the  great  events 
of  the  earth's  history  in  chronological  order. 

The  general  principle  that  similarity  of  fossils  indicates  the  ap- 
proximate contemporaneity  of  the  rocks  in  which  they  are  found, 
must  not  be  taken  too  literally,  for  it  is  subject  to  certain  limita- 
tions and  exceptions. 

(a)  Exact  contemporaneity  is  not  meant,  for  the  progress  of 
life  is  very  slow,  and  rocks  formed  thousands  of  years  apart  may 
yet  contain  precisely  similar  fossils. 

(<£)  Animals  and  plants  differ  in  different  parts  of  the  world,  so 
that  contemporaneous  rocks  formed  in  widely  separated  regions 
will  always  show  a  certain  amount  of  difference  in  their  contained 
fossils.  In  comparing  the  rocks  of  two  continents,  it  is  often 
exceedingly  difficult  to  decide  just  how  much  of  a  given  difference 
in  the  fossils  is  to  be  ascribed  to  a  difference  in  the  time  of  rock 
formation,  and  how  much  to  mere  geographical  separation. 

(<r)  New  forms  of  animals  and  plants  originate  in  some  particu- 
lar area  and  spread  in  all  directions  from  that  area,  until  stopped 
by  some  obstacle  of  climate  or  topography  which  they  cannot  sur- 
mount. The  diffusion  of  new  forms  often  occasions  the  extinction 
of  old  ones  which  were  not  so  well  fitted  to  survive.  These  pro- 
cesses take  time,  and  a  group  of  organisms  may  make  its  appearance 
in  one  part  of  the  world  long  before  it  spreads  to  another,  while 
ancient  types  may  linger  in  certain  localities  long  after  they  have 
elsewhere  become  extinct. 


GEOLOGICAL  CHRONOLOGY  349 

Despite  these  limitations  we  find  that,  speaking  broadly,  the 
order  of  succession  in  the  appearance  and  extinction  of  the  great 
groups  of  fossils  is  much  the  same  for  all  parts  of  the  earth,  and  we 
may  confidently  assume  that  the  grander  divisions  of  geological 
time  are  of  world-wide  significance. 

It  will  now  be  easy  to  understand  why  the  fossils  in  two  groups 
of  unconformable  strata  are  generally  so  radically  different.  It  is 
because  of  the  long  lapse  of  unrecorded  time  at  that  point,  during 
which  organic  progress  continued  ;  when  deposition  was  resumed, 
the  animals  and  plants  were  all  new,  and  so  the  change  is  abrupt. 
"'If  one  is  reading  a  book  from  which  a  dozen  chapters  have  been 
torn  out,  the  change  of  subject  will  appear  violently  abrupt ;  to 
bridge  over  the  gap  one  must  find  another  copy  of  the  book/' 
Likewise,  to  fill  up  the  gap  of  a  great  unconformity,  we  must  go 
to  some  region  where  deposition  went  on  uninterruptedly,  and 
there  we  may  trace  the  gradual  and  steady  change  in  the  fossils. 

A  geological  chronology  is  constructed  by  carefully  determining, 
first  of  all,  the  order  of  superposition  of  the  stratified  rocks,  and 
next  by  learning  the  fossils  characteristic  of  each  group  of  strata. 
The  history  is  recorded  partly  in  the  nature  and  structure  of  the 
rocks,  partly  in  the  fossils,  and  partly  in  the  topographical  forms  of 
the  land  and  the  courses  of  the  streams.  By  combining  these 
different  lines  of  evidence,  local  histories  are  constructed  for  each 
region,  until  from  these  the  story  of  the  whole  continent  may  be 
compiled.  The  comparative  study  of  the  fossils  then  gives  the 
clue  for  uniting  the  history  of  the  different  continents  into  the 
history  of  the  earth.  Much  remains  to  be  done  before  this  great 
task  can  be  accomplished,  but  already  we  have  an  outline  of  the 
scheme  which  future  investigations  may  fill  up. 

It  necessarily  follows  from  the  way  in  which  sedimentary  rocks 
are  formed,  and  the  local  nature  of  upheavals  and  depressions  of 
land,  that  in  no  single  locality  can  the  entire  series  of  strata  be 
observed,  and  that  each  region  can  display  but  a  certain  propor- 
tion of  the  whole  record.  The  different  parts  of  our  continent 
are  of  vastly  different  geological  dates,  and  even  the  same  area 
may  have  been  many  times  a  land  surface,  and  as  often  a  sea- 


350  FOSSILS 

bottom.  Unconformities,  more  or  less  wide-spread,  offer  a  natural 
and  convenient  mode  of  dividing  the  strata  into  groups,  but  the 
difficulty  with  this  method  is  that  the  dates  of  elevation  and  de- 
pression so  seldom  correspond  in  different  regions,  that  divisions 
thus  made  are  apt  to  be  of  more  or  less  local  validity.  The  only 
standard  yet  devised  which  is  applicable  to  all  the  world  is  that 
founded  upon  the  progress  of  life. 

The  comparison  between  human  history  and  geological  history 
is  one  that  has  very  often  been  made,  but  trite  and  hackneyed  as 
it  is,  it  is  none  the  less  instructive.  The  history  of  civilized  na- 
tions is  the  record  of  continuous  development,  not  without  retro- 
gressions and  periods  of  comparative  stagnation,  but  having  no 
actual  gaps  in  it.  For  the  sake  of  convenience,  history  is  divided 
into  certain  periods  in  accordance  with  the  predominance  of  cer- 
tain great  ideas  and  principles,  and  these  periods  are  real,  repre- 
senting the  salient  facts  in  the  progress  of  development.  Each 
period  is,  however,  but  the  outcome  of  the  antecedent  periods, 
and  the  ideas  and  principles  which  characterize  it  were  slowly 
maturing,  it  may  be  through  centuries,  and  even  after  other  ideas 
have  risen  to  predominance,  older  ones  continue  to  live  and  influ- 
ence the  world.  For  example,  when  we  speak  of  the  age  of  the 
French  Revolution,  we  refer  to  a  time  when  a  certain  set  of  politi- 
cal ideas  and  principles  were  the  most  striking  and  influential 
factors  in  the  development  of  the  civilized  world,  beginning  with 
the  visible  changes  of  1789  and  ending  with  the  restoration  of  the 
Bourbons  in  1815.  But  the  tremendous  outbreak  was  slowly  pre- 
paring throughout  the  Eighteenth  Century ;  the  conflagration  was 
proportionate  to  the  materials  that  had  been  gathered  for  it.  Nor, 
on  the  other  hand,  could  the  effects  of  the  great  movement  be 
undone  by  the  return  of  the  exiled  king.  To  this  day  the  whole 
civilized  world  feels  the  effects  of  the  convulsion,  and  the  entire 
course  of  the  Nineteenth  Century  would  have  been  different  but 
for  the  French  Revolution. 

Historians  are  careful  to  distinguish  between  events  and  the 
record  of  them.  Events  are  continuous  and  bound  up  into  a 
chain  of  consequences,  every  one  of  which  is  dependent  upon 


CATASTROPHISM  3  5  I 

others,  while  the  records  may  -be  scanty,  interrupted,  confused, 
unintelligible,  even  misleading  and  falsified,  so  that  it  is  no  easy 
task  to  write  history  accurately  and  without  attributing  undue  im- 
portance to  this  or  that  principle  or  policy. 

These  considerations  fully  apply  to  geological  history ;  its  divi- 
sions are  founded  upon  the  rise  and  culmination  of  great  groups 
of  animals  and  plants,  which  one  after  another  have  risen  to  pre- 
dominance and  then  declined,  their  place  being  taken  by  others 
better  fitted  for  the  new  conditions.  These  successive  culminations 
are  not  sudden,  but  gradual  and  continuous,  and  the  beginnings 
of  each  group  are  to  be  found  in  time  long  before  the  period  of 
its  predominance.  Nor  is  decline  immediately  followed  by  extinc- 
tion; one  group  slowly  gives  way  to  another,  but  long  after  the 
first  has  ceased  to  be  the  principal  fact  in  the  world's  life,  it  may 
linger  on  in  diminished  importance  until,  perhaps,  it  finally  dis- 
appears. The  geological  periods,  therefore,  like  historical  periods, 
had  not  definite  beginnings  and  endings,  for  one  slowly  fades  into 
another,  but  they  are  none  the  less  actual  because  the  lines  of 
separation  between  them  must  often  be  somewhat  arbitrarily 
drawn,  and  they  cannot  always  be  made  to  correspond  in  differ- 
ent regions. 

In  geology,  as  in  history,  we  must  distinguish  between  the  events 
and  the  records  of  them.  The  more  complete  the  records,  the 
more  obviously  continuous  and  gradual  was  the  course  of  events ; 
only  imperfect  records  can  make  the  history  seem  broken  and  dis- 
jointed. As  our  science  was  first  developed  in  western  Europe, 
where  the  great  groups  of  strata  are  mostly  separated  by  uncon- 
formities, with  abrupt  changes  in  the  fossils,  the  older  geologists 
very  naturally  concluded  that  the  great  divisions  of  geological  time 
were  marked  by  frightful  catastrophes  which  devastated  the  earth, 
destroying  every  living  thing  upon  it.  'teach  group  of  rocks  was 
looked  upon  as  the  product  of  a  long  and  tranquil  period,  and  its 
fossils  were  believed  to  represent  an  entirely  new  creation/'  Though 
opposed  by  some  far-seeing  minds,  the  doctrine  of  Catastrophism, 
as  it  was  called,  long  held  sway,  but  was  shown  to  be  erroneous, 
when  the  study  of  geology  was  carried  to  other  parts  of  the  world. 


352  FOSSILS 

Then  it  appeared  that  the  supposed  catastrophes,  if  they  occurred 
at  all,  were  not  general,  but  local,  that  records  missing  in  Eu- 
rope had  been  preserved,  partially  at  least,  in  other  continents. 
Enough  of  these  missing  records  has  been  recovered  to  show  that 
the  earth's  progress  was  not  by  a  series  of  abrupt  and  sudden 
changes,  but  by  a  continuous,  orderly  development. 

Major  divisions  of  geological  time  are  founded  upon  the  more 
striking  changes  in  the  animals  and  plants,  while  for  minor  divi- 
sions the  more  detailed  differences  in  the  organisms  are  employed. 
Parallel  with  the  divisions  of  time  run  the  groups  or  systems  of 
the  strata,  for  characterizing  which  both  the  physical  nature  and 
structure  of  the  rocks  and  the  fossils  are  employed.  In  the  very 
difficult  and  complicated  task  of  compiling  the  earth's  history,  no 
kind  of  evidence  can  be  ignored,  and  wide  knowledge  and  sound 
judgment  are  needed  in  the  work,  so  that  no  particular  class  of 
records  shall  be  either  over-  or  under-valued. 

Though  the  goal  of  geological  inquiries  is  to  construct  the  his- 
tory of  the  earth  as  a  unit,  this  goal  can  only  be  reached  by  the 
minute  and  exhaustive  study  of  the  local  histories.  Each  of  the 
latter  has  certain  peculiarities  of  its'  own  which  must  be  deter- 
mined, and  hence  arises  the  multiplicity  of  local  names  for  groups 
of  strata,  so  confusing  to  the  student.  Local  names  are  useful, 
because  they  avoid  the  necessity  of  premature  comparisons  and 
correlations,  which  may  lead  to  the  direst  mistakes. 

(2)  As  Evidence  of  Geographical  Changes. — We  have  seen 
that  from  the  composition  and  structure  of  the  stratified  rocks 
themselves  much  may  be  learned  concerning  the  geographical 
conditions  under  which  they  were  formed,  and  of  the  subsequent 
geographical  changes  of  the  region  in  which  they  occur.  Fossils 
supplement  this  information  regarding  the  body  of  water  in  which 
the  rocks  were  laid  down,  whether  fresh  or  salt,  deep  or  shallow, 
near  or  far  from  land,  in  an  open  sea  or  a  closed  basin,  and  whether 
such  a  closed  basin  had  occasional  or  constant  communication 
with  the  ocean.  The  stratified  rocks  which  now  form  part  of  the 
land  surface  give  us  information  only  concerning  the  former  ex- 
tension of  bodies  of  water  over  what  is  now  the  land,  but  they  can 


CLIMATIC   CHANGES  353 

tell  us  nothing  of  the  land  areas  which  have  disappeared  beneath 
the  sea.  In  this  connection  fossils  are  of  great  assistance,  for,  in 
certain  instances,  the  distribution  of  marine  fossils  points  to  the 
presence  of  land  barriers  to  migration  which  no  longer  exist,  while 
the  fossils  of  land  animals  may  demonstrate  the  former  existence 
of  land  bridges  between  regions  which  have  long  been  separated 
by  water.  '''Thus  it  may  be  shown  that  North  America  was  fre- 
quently and  for  long  periods  of  time  connected  with  Asia  across 
Bering's  Sea,  and  that  its  union  with  South  America  is  of  geologi- 
cally late  date.  * 

(3)  As  Evidence  of  Climatic  Changes. — The  remarkable  cli- 
matic changes  through  which  various  parts  of  the  earth  have 
passed  are  indicated  by  fossils.  Indeed,  with  the  exception  of 
glacial  marks,  and  ice-formed  deposits,  fossils  offer  almost  the 
only  trustworthy  evidence  available  as  to  these  changes  of  climate. 
Thus,  when  we  find  in  the  rocks  of  Greenland  the  remains  of  ex- 
tensive forests  of  such  trees  as  now  grow  in  temperate  latitudes, 
the  only  possible  inference  is  that  Greenland  now  has  a  far  colder 
climate  than  when  those  forests  existed.  The  same  conclusion 
follows  from  the  presence  in  the  rocks  of  Wyoming  and  Idaho  of 
great  palm  leaves  and  other  subtropical  plants  associated  with  the 
bones  of  crocodiles  and  other  reptiles,  such  as  live  only  in  warm 
regions.  In  deposits  of  a  far  later  date  occur  bones  of  the  rein- 
deer in  southern  New  England  and  in  the  south  of  France,  walrus 
bones  in  the  sands  of  New  Jersey,  and  those  of  the  musk-ox  in 
Arkansas ;  all  of  which  shows  that  at  one  time  these  regions  had  a 
far  colder  climate  than  at  present. 

The  evidence  as  to  climatic  changes  which  is  presented  by  fos- 
sils must,  however,  be  treated  with  great  caution,  because  even 
nearly  allied  species  often  have  entirely  different  habits  and 
flourish  in  quite  different  climates.  Most  fossils  belong  to  extinct 
species,  as  to  whose  climatic  relations  we  have  no  knowledge. 
Before  any  conclusion  concerning  changes  of  climate  can  be  re- 
garded as  established,  we  should  have  the  testimony  of  species 
still  living,  or,  if  that  is  not  possible,  the  evidence  must  be  drawn 
from  large  assemblages  of  different  kinds  of  animals  and  plants. 
2  A 


354  FOSSILS 

Such  an  extreme  case  as  the  fossil  plants  of  Greenland  is  sufficient 
evidence  without  further  corroboration. 


III.   CLASSIFICATION  OF  GEOLOGICAL  TIME 

The  method  of  making  the  divisions  and  subdivisions  of  geo- 
logical time  is  not  yet  a  fixed  one,  and  there  is  much  difference  in 
the  usage  of  various  writers.  The  names  of  the  divisions  also 
have  been  given  at  various  times  and  in  many  lands,  according  to 
no  particular  system.  Most  of  these  names  have  been  taken  from 
the  locality  or  district  where  the  rocks  in  question  were  first 
studied ;  as  Devonian  from  Devonshire,  Jurassic  from  the  Jura 
Mountains.  Some  are  named  from  a  characteristic  or  prevalent 
kind  of  rock,  such  as  Cretaceous  (Latin  creta,  chalk)  and  Carbonif- 
erous. Of  late  there  has  been  a  tendency  toward  a  more  uniform 
method  of  nomenclature,  and  to  the  use  of  one  set  of  terms  for 
the  divisions  of  time,  and  another  and  corresponding  set  for  the 
divisions  of  the  strata.  The  grander  divisions  of  time  are  called 
eras,  and  in  descending  order  we  have  periods,  epochs,  and  ages. 
The  following  table  represents  the  divisions  in  the  scale  of  time 
and  the  scale  of  rocks  which  have  been  adopted  by  the  Inter- 
national Geological  Congress. 

TIME  SCALE  ROCK  SCALE 

Era  Group 

Period  System 

Epoch  Series 

Age  Stage 

Substage 

Zone 

It  will  be  observed  that  the  subdivision  is  carried  further  in  the 
scale  of  rocks  than  in  that  of  time,  because  of  the  generally  local 
character  of  these  minor  subdivisions.  The  names  employed  are, 
as  yet,  the  same  for  both  scales,  and  we  speak  of  the  Palaeozoic 
Era  or  Group,  and  of  the  Silurian  Period  or  System.  It  has  been 
proposed  to  give  separate  names  to  the  divisions  of  the  two  scales, 


SUBDIVISIONS   OF  TIME 


355 


and  this  would  be  an  improvement  in  some  respects,  but  the  pro- 
posal has  not  been  carried  out. 

TABLE  OF  GEOLOGICAL  DIVISIONS 


Cenozoic  Era 


Mesozoic  Era 


Quaternary  Period  or  Pleistocene  Epoch 
Pliocene  Epoch 


Tertiary 


Cretaceous  Period1 
Jurassic  " 

Triassic  " 


Miocene       " 

Oligocene    " 

.  Eocene        " 


Palaeozoic  Era 


Eozoic  Era 
Azoic  Era 


Permian         Period 
Carboniferous    " 

Devonian  " 

Silurian  " 

Ordovician         " 

Cambrian  " 

Algonkian  Period 
Archaean  Period 


Upper  Carbonif.  Epoch 

Lower  Carbonif.  " 
(  Chemung 
I  Hamilton 

I  Corniferous  " 

I  Oriskany  " 

Lower  Helderberg   " 

Onondaga  " 

Niagara  " 

Canadian  " 

Trenton  " 

f  Potsdam  " 

I  Acadian  " 

[  Georgian  " 


l  The  subdivisions  of  the  Mesozoic  Periods  are  so  different  for  different  parts  of 
the  continent  that  they  are  omitted  in  the  table. 


CHAPTER   XXI 

ORIGINAL  CONDITION  OF   THE  EARTH  — PRE-CAMBRIAN 
PERIODS 

As  we  trace  the  history  of  mankind  back  to  very  ancient  times, 
we  find  that  the  records  become  more  and  more  scanty  and  less 
intelligible,  until  history  fades  into  myth  and  tradition.  Of  a 
still  earlier  age  we  have  not  even  a  tradition ;  it  is  prehistoric. 
Similarly,  among  the  geological  records  the  earliest  are  in  a 
state  of  such  excessive  confusion  that  they  are  exceedingly  diffi- 
cult to  understand,  and  between  different  observers  there  are 
radical  differences  of  opinion  both  as  to  the  facts  and  as  to  their 
interpretation.  Furthermore,  there  must  have  been  an  inconceiv- 
ably long  time  earlier  than  the  most  ancient  recorded  periods,  as 
to  which  conjecture  and  inference  are  the  only  resource.  In  these 
difficult  straits  astronomy  offers  valuable  assistance  to  the  baffled 
geologist.  The  Nebular  Hypothesis  is  a  scheme  of  the  develop- 
ment of  the  solar  system,  which,  though  not  yet  demonstrated,  nor 
free  from  difficulties,  so  well  explains  the  known  facts  that  it  is 
almost  universally  accepted  as  essentially  true. 

According  to  this  hypothesis  the  place  of  the  present  solar  system 
was  originally  occupied  by  a  vast  rotating  nebula,  a  mass  of  in- 
tensely heated  vapour,  or  possibly  clouds  of  meteorites,  extending 
beyond  the  orbit  of  the  outermost  planet.  As  the  nebula  cooled 
by  radiation,  it  contracted,  leaving  behind  it  successive  rings,  like 
those  of  the  planet  Saturn,  but  on  a  vastly  larger  scale.  The  rings 
kept  up  the  relation  imparted  by  the  nebula,  and  all  of  them  lay 
in  nearly  the  same  plane.  Unequal  contraction  in  various  parts  of 
each  revolving  ring  caused  it  to  break  up  and  gather  by  mutual 
attraction  into  masses.  If  these  rings  were  composed  of  relatively 
small  solid  masses,  like  meteorites,  or  if  they  had  solidified  by  con- 

356 


ASTRAL   PERIOD  357 

densation  of  the  vapours,  the  heat  generated  by  the  collisions,  as 
the  broken  ring  was  gathered  into  a  mass,  would  suffice  to  raise 
the  temperature  and  liquefy  or  vapourize  the  mass.  By  revolution 
the  nebulous  masses  would  assume  a  spheroidal  shape  and  become 
planets.  The  central  mass  of  the  original  nucleus  forms  the  sun, 
which  is  still  in  an  intensely  heated,  incandescent  state. 

This  is  not  the  place  to  discuss  the  evidence  for  an  astronomical 
speculation,  but  we  may  regard  it  as  in  the  highest  degree  probable 
that  when  our  earth  first  began  its  separate  existence,  it  did  so  as 
a  globe  of  fused  or  even  vaporous  material,  in  which  the  various 
substances  arranged  themselves  very  much  in  the  order  of  their 
density.  This  conclusion  is  much  strengthened  by  the  character 
of  the  globe  itself.  The  specific  gravity  of  the  earth  as  a  whole 
somewhat  exceeds  5,  while  that  of  the  rocks  on  the  surface  ranges 
from  2.5  to  3,  which  shows  that  the  interior  of  the  earth  is  much 
denser  than  its  outer  portion.  We  have  already  learned  that  the 
earth's  interior  is  still  highly  heated,  and  its  shape,  that  of  an 
oblate  spheroid  flattened  at  the  poles,  is  that  which  would  be 
assumed  by  a  rotating  liquid  or  plastic  body. 

For  long  ages  the  earth  must  have  continued  as  a  glowing  star,  its 
"  astral  period"  but  gradually  an  outer  crust  was  formed  by  the  slow 
cooling  of  the  surface.  How  often  this  thin  crust  was  broken  up 
and  remelted  and  formed  again,  we  have  no  means  of  knowing,  but 
eventually  a  solid,  permanent  crust  was  established  and  thickened 
by  additions  from  below,  until,  by  the  combined  effects  of  cooling 
and  enormous  pressures,  the  globe  has  reached  its  present  condi- 
tion, whatever  that  may  be  (see  p.  32).  Even  after  a  solid  crust 
had  been  formed,  it  must  have  long  remained  so  hot  that  no  water 
could  condense  upon  its  surface.  All  the  water  of  the  oceans  must 
then  have  been  in  the  atmosphere,  increasing  many  fold  the  weight 
of  the  latter  and  the  pressure  which  it  exerted  upon  the  earth's  sur- 
face. Owing  to  this  great  pressure  the  first  condensation  of  steam 
must  have  occurred  at  temperatures  far  above  the  boiling-point  of 
water  for  the  present  atmospheric  pressure  (21 2°  F.).  Superheated 
water  is  an  agency  of  extreme  power  in  destroying  and  recombining 
materials,  and  the  earliest  boiling  oceans  must  have  set  up  very 


358  PRE-CAMBRIAN    PERIODS 

vigorous  chemical  work  upon  the  heated  crust.  In  the  course  of 
ages  the  surface  of  the  globe  became  so  far  cooled  and  the  crust 
so  thick  that  the  earth's  interior  heat  ceased  to  control  the  tem- 
perature of  its  surface. 

THE  PRE-CAMBRIAN  PERIODS  — I.   ARCHAEAN 

It  is  unfortunate  that  an  account  of  historical  geology  should 
necessarily  begin  with  the  most  difficult  and  obscure  part  of 
the  whole  subject,  but  the  treatment  must  be  in  accordance 
with  the  chronological  order,  and  the  oldest  rocks  are  the  least 
intelligible.  The  ordinary  criteria  of  the  historical  method, 
namely,  the  stratigraphical  succession  and  the  comparison  of 
fossils,  fail  us  here  almost  entirely,  and  the  only  way  of  corre- 
lating the  rocks  of  different  regions  and  continents  is  by  means 
of  the  characters  of  the  rocks  themselves.  In  the  present  state 
of  knowledge,  "  lithological  similarity  "  is  not  a  safe  guide.  So 
many  metamorphic  rocks,  once  referred  to  the  Archaean,  have 
proved  to  be  of  much  later  date,  that  some  cautious  geologists, 
who  have  no  confidence  in  "  lithological  similarities,"  prefer  not 
to  use  the  term  Archcean  at  all,  but  to  employ  local  terms  for 
the  oldest  crystalline  rocks  exposed  in  a  given  district. 

The  Archaean  includes  the  most  ancient  rocks,  often  spoken  of 
as  the  "  basement,  or  basal  complex."  Its  antiquity  is  best  as- 
sured in  regions  where  it  is  separated  by  thick  series  of  sediment- 
ary or  metamorphic  rocks  from  the  Lower  Cambrian,  which  can 
be  certainly  identified  by  its  fossils.  In  such  regions  the  Archaean 
is  composed  of  completely  crystalline  rocks  of  various  types  con- 
fusedly mixed  together,  massive  rocks,  such  as  granite  and  basic 
eruptives,  and  foliated  rocks,  like  gneissoid  granite,  gneiss,  and 
various  schists,  are  intermingled  in  the  most  intricate  way.  Some 
of  these  rocks  cut  the  others  in  the  form  of  dikes  and  are  mani- 
festly of  very  different  dates  of  formation.  The  dike  rocks  may  be 
either  massive  or  schistose.  The  component  minerals  are  princi- 
pally orthoclase  and  acid  plagioclase,  quartz,  hornblende,  and  mica, 
with  other  minerals  as  accessories.  The  particles  show  plainly  the 


DISTRIBUTION   OF  ARCH^AN   ROCKS  359 

intense  dynamic  metamorphism  to  which  they  have  been  subjected, 
in  their  extremely  complex  arrangement  and  in  their  laminated 
and  crushed  condition.  The  Archaean,  as  thus  defined,  contains  no 
sandstone,  conglomerate,  limestone,  or  any  other  rock  of  undoubted 
sedimentary  origin,  nor  any  considerable  mass  of  quartz  schist, 
marble,  or  graphite  schist.  The  rocks  thus  referred  to  the  Archaean 
are  not  necessarily  all  of  the  same  age,  but  they  are  all  of  vast 
antiquity  and  older  than  any  other  known  series.  They  are  of  very 
great  but  unknown  thickness,  for  the  bottom  of  them  is  nowhere 
to  be  seen,  and  even  when  thrown  up  into  mountain  ranges,  ero- 
sion has  in  no  case  cut  so  deeply  into  these  rocks  as  to  expose 
anything  different  below  them. 

The  reason  for  uniting  these  rocks  into  one  group  is  not  merely 
their  likeness  in  composition,  which  is  not  a  sufficient  criterion, 
but  because  of  their  unique  and  uniformly  complex  structure, 
their  resemblance  to  one  another  and  difference  from  any  other 
group  of  rocks,  and  their  invariably  fundamental  position. 

The  Distribution  of  the  Archaean  Rocks  can  at  present  be 
stated  only  with  much  reserve,  for  they  often  grade  into  crystal- 
line schists  of  demonstrably  later  date,  and  much  that  once  was 
referred  to  the  Archaean  is  now  known  to  be  far  more  recent. 
To  accurately  determine  the  distribution  of  the  basal  complex 
will  require  the  most  extensive,  minute,  and  laborious  investiga- 
tion. The  northern  part  of  North  America,  from  the  Arctic 
Ocean  to  the  Great  Lakes,  is  made  up  of  an  immense  area  of 
schistose  rocks,  estimated  at  more  than  2,000,000  square  miles  in 
extent.  Over  this  vast  region  occur  numerous  areas  of  Archaean 
rocks,  but  it  is  not  yet  possible  to  say  how  much  of  it  belongs  in 
that  group  and  how  much  is  newer. 

Beside  this  principal  region  are  several  other  minor  ones.  A 
narrow  band  of  schistose  rocks  extends,  with  some  interruptions, 
from  Vermont  to  Georgia,  with  shorter  parallel  belts  in  eastern 
Canada  and  New  England.  Another  great  axis  is  on  the  site  of 
the  Rocky  Mountain  chain,  with  several  shorter  and  generally 
parallel  belts  from  Mexico  to  Alaska.  Isolated  areas  occur  in 
Missouri,  central  Texas,  New  Mexico,  and  Arizona.  In  all  of 


360  PRE-CAMBRIAN   PERIODS 

these  regions  are  found  rocks  like  the  typical  Archaean,  which 
stand  in  the  same  relation  to  the  newer  groups,  but  how  much 
should  be  referred  to  these  newer  groups  is  still  a  question. 

In  the  other  continents  occur  great  areas  of  very  ancient  gneisses 
and  crystalline  schists,  but  even  less  than  in  North  America  has  the 
distinction  been  made  between  the  fundamental  complex  and  newer 
groups.  In  the  following  statements  no  attempt  is  made  to  deter- 
mine how  much  of  the  areas  mentioned  is  properly  Archaean. 

In  Europe  the  principal  area  lies  to  the  north,  covering  parts  of 
Ireland  and  the  Highlands  of  Scotland,  with  which  was  probably 
once  connected  the  great  continuous  mass  of  Scandinavia,  Fin- 
land, and  Lapland.  Considerable  areas  also  occur  in  central 
and  southern  Europe,  as  the  central  plateau  of  France,  parts  of 
Germany  and  Bohemia,  and  long,  narrow  belts  in  the  Pyrenees, 
Alps,  and  Balkans.  In  Asia  these  ancient  crystalline  rocks  are 
found  in  the  great  mountain  ranges,  such  as  the  Himalayas,  Altai, 
etc.  They  make  up  a  large  part  of  the  Indian  peninsula,  and  are 
extensively  displayed  in  China,  Japan,  and  the  islands  of  the  Malay 
Archipelago.  The  vast  central  plateau  which  occupies  so  much 
of  Africa  is  principally  composed  of  these  rocks,  which  are  also 
largely  exposed  in  Australia.  In  South  America  similar  rocks 
appear  in  the  highlands  of  Brazil  and  in  the  Andes. 

Origin  of  the  Archaean  Rocks.  —  This  is  a  problem  which  has 
given  rise  to  a  great  deal  of  discussion  and  is  still  far  from  solu- 
tion. One  reason  for  the  great  differences  of  opinion  is  the 
varying  extension  which  has  been  given  to  the  term  Archcean, 
one  writer  including  rocks  which  another  excludes.  The  principal 
suggestions  which  have  been  offered  are  the  following :  — 

(1)  That  the  Archaean  rocks  are  entirely  of  igneous  origin  and 
represent  part  of  the  original  crust  of  the  earth,  added  to  from 
below  by  solidification  and  cut  by  many  subsequent  igneous  intru- 
sions.    On  this  view  these  rocks  are  far  older  than  any  sediment- 
aries  whatever. 

(2)  That  the  Archaean  rocks  were  precipitated  from  solution 
in  the  hot  seas  which  first  condensed,  under  great   atmospheric 
pressures,  upon  the  highly  heated  crust. 


ALGONKIAN  361 

(3)  That  these  rocks  are  intrusive  igneous  masses,  newer  than 
certain  strata  which  rest  upon  them. 

(4)  That  they  were   formed    by  the    metamorphism    of  sedi- 
mentary rocks,  the  massive  kinds  representing,  in  large  part,  the 
extreme   stage  of  metamorphism  by  which   the  sediments  were 
actually  melted.     Others  of  the  massive  kind  are  igneous  intru- 
sions of  all  subsequent  dates. 

While  we  have,  as  yet,  no  means  of  definitely  deciding  among 
these  conflicting  opinions,  yet  the  present  trend  of  investigation 
seems  to  be  distinctly  in  favour  of  the  first  view,  or  some  modifi- 
cation of  it.  Certain  it  is  that,  if  the  original  crust  of  the  earth  be 
anywhere  preserved,  it  is  in  the  Archaean  rocks.  As  these  have 
been  subjected  to  all  the  folding  and  crushing  which  the  earth's 
crust  has  undergone,  it  is  not  surprising  that  they  should  have 
acquired  such  a  complex  and  intricate  structure  and  have  been  so 
radically  metamorphosed.  It  is  hardly  necessary  to  say  that  the 
Archaean,  as  here  limited,  has  yielded  no  evidence  of  life,  all  of 
those  evidences  which  are  generally  spoken  of  as  found  in  the 
Archaean  being  of  later  date,  but  this  negative  testimony  is  of  no 
great  value.  If  these  rocks  be  actually  transformed  sediments, 
the  profound  metamorphism  which  they  have  undergone  would 
have  thoroughly  destroyed  any  traces  of  fossils  that  they  might 
have  originally  contained.  We  cannot  tell  when  life  was  first 
introduced  upon  the  earth,  but  we  may  be  very  confident  that  no 
living  thing  could  have  existed  when  the  surface  of  the  crust  was 
glowing  hot,  or  in  the  oceans  boiling  even  under  the  enormous 
atmospheric  pressures  which  accompanied  their  first  condensation. 


II.   ALGONKIAN 

This  is  the  name  recently  proposed  by  the  United  States  Geo- 
logical Survey  for  the  great  series  of  sedimentary  and  metamorphic 
rocks  which  lie  between  the  basal  Archaean  complex  and  the  oldest 
Palaeozoic  strata.  While  it  is  possible,  though  not  very  likely,  that 
more  advanced  knowledge  may  lead  us  to  distribute  these  rocks 
partly  into  the  Archaean  and  partly  into  the  Palaeozoic,  yet  for 


362  PRE-CAMBRIAN   PERIODS 

the  present,  at  least,  it  is  better  to  form  a  separate  grand  division 
for  them. 

The  Algonkian  rocks,  which  are  widely  distributed  in  North 
America,  form  an  immensely  thick  mass  of  strata  and  of  metamor- 
phic  rocks  which  are  believed  to  represent  the  strata.  These 
metamorphic  rocks  have  hitherto  been  generally  referred  to  an 
upper  division  of  the  Archaean,  called  the  Huronian,  but,  so  far 
as  can  be  learned,  they  occupy  the  same  stratigraphical  position 
as  certain  little  changed  sediments,  between  the  fundamental  com- 
plex below  and  the  Cambrian  above.  At  the  base  of  the  magnifi- 
cent section  exposed  in  the  Grand  Canon  of  the  Colorado  is  a 
very  thick  mass  of  strata,  separated  by  great  unconformities  from 
the  Archaean  gneiss  below  and  from  the  overlying  Upper  Cam- 
brian. This  mass  is  again  subdivided  by  minor  unconformities 
into  three  series.  The  lower  series,  at  least  1000  feet  thick  and 
perhaps  more,  is  made  up  of  stratified  quartzites  and  semi-crystal- 
line schists,  cut  by  intrusive  granite.  Above  this  come  nearly 
7000  feet  of  sandstones,  with  included  lava  sheets,  and  at  the  top 
more  than  5000  feet  of  shales  and  limestones,  in  which  a  few  fos- 
sils have  been  found.  The  two  upper  series  are  not  at  all  meta- 
morphic. All  these  strata  are  steeply  inclined,  and  upon  their 
edges  rests  the  Upper  Cambrian. 

A  very  similar  succession  of  rocks  of  vast  thickness  is  found 
in  the  Lake  Superior  region,  intervening  between  the  Archaean 
complex  and  the  Upper  Cambrian,  from  both  of  which  they  are 
separated  by  great  unconformities.  As  in  the  Grand  Canon 
section,  these  rocks  are  divisible  into  three  series  by  minor  un- 
conformities. The  lowest,  with  a  maximum  thickness  probably 
exceeding  5000  feet,  is  much  crumpled,  metamorphosed,  and 
semi-crystalline.  It  comprises  limestones,  quartzites,  mica  schists, 
etc.,  cut  by  igneous  dikes,  also  much  volcanic  tuff  and  agglomerate. 
Next  follows  a  series  of  12,000  feet  of  less  intensely  folded 
but  still  metamorphic  rocks,  quartzites,  shales,  slates,  mica  schists, 
with  dikes  and  interbedded  sheets  of  diorite.  A  few  fossils  have 
been  found  in  the  quartzites  of  this  series.  The  third  series  has 
a  maximum  thickness  of  50,000  feet,  though  usually  much  less. 


ALGONKIAN    LIFE  363 

The  lower  part  of  this  series  is  formed  by  thick  lava  sheets,  inter- 
bedded  with  sandstone  and  conglomerate,  and  above  is  a  mass  of 
sedimentary  rocks  largely  derived  from  the  volcanic  materials. 

Over  the  great  Archaean  area  of  Canada  occur  many  districts 
of  metamorphic  rocks  which  are  plainly  of  sedimentary  origin, 
such  as  crystalline  limestones,  schistose  conglomerates,  as  well 
as  volcanic  tuffs  and  agglomerates.  In  this  region  and  in  New 
England  the  Algonkian  metamorphics  apparently  grade  into  the 
Archaean  complex  without  unconformity.  This  apparent  conformity 
may,  however,  very  well  be  due  to  subsequent  dynamic  metamor- 
phism,  which,  as  has  been  proved,  may  obliterate  nearly  all  traces 
of  a  great  unconformity.  Through  the  Rocky  Mountain  region  and 
the  Pacific  coast  mountains,  the  Archaean  is  in  very  many  places 
overlaid  by  great  thicknesses  of  metamorphic  Algonkian  rocks, 
such  as  quartzites,  sandstones,  and  schists,  which  are  sometimes  as 
much  as  12,000  feet  thick,  as  in  the  Wasatch  and  Uinta  mountains. 
Other  isolated  areas  are  found,  as  in  the  Black  Hills,  where  a  great 
mass  of  schists,  slates,  and  quartzites  is  separated  by  a  very  marked 
unconformity  from  the  overlying  Cambrian ;  also  in  Missouri  and 
Texas.  The  Algonkian  rocks  of  the  West  have  not  been  subjected 
to  such  extreme  folding  as  have  those  of  the  East,  and  hence 
their  distinctness  from  the  Archaean  is  more  clearly  marked. 

In  other  continents  the  distinction  has  hardly  been  drawn  yet 
between  the  Archaean  and  the  Algonkian.  In  Great  Britain,  how- 
ever, are  found  very  interesting  parallels  with  the  Algonkian  of  this 
country.  In  Scotland  the  Torridon  sandstones,  8000  to  10,000 
feet  thick,  which  are  nearly  horizontal  and  almost  unchanged, 
lie  unconformably  between  the  oldest  Cambrian  and  the  basal 
Archaean ;  and  in  other  areas,  metamorphic  rocks  of  sediment- 
ary origin  occupy  a  similar  position.  Many  of  the  crystalline 
schists  of  the  European  pre-Cambrian  areas  appear  to  correspond 
in  character  and  position  to  the  metamorphic  Algonkian. 

Life  in  the  Algonkian.  —  In  the  Grand  Canon  and  the  Lake 
Superior  region  determinable  fossils  have  been  found  in  the  less 
changed  sediments,  but  they  are  too  few  and  scanty  to  tell  us 
much  of  the  life  of  the  times.  It  must  also  be  remembered  that 


364  PRE-CAMBRIAN    PERIODS 

the  rocks  in  which  they  occur  may  eventually  prove  to  be  Cam- 
brian. Evidences  of  life  are  not  wanting  in  the  metamorphic 
rocks  of  the  eastern  and  northern  regions,  but  they  are  indirect 
and  not  entirely  conclusive.  The  strata  of  crystallized  limestone 
are  indications  of  the  presence  of  animal  life  in  the  Algonkian 
seas,  for  the  only  way  in  which  such  masses  of  limestone  can  be 
formed  now  is  by  the  organic  agencies  (see  Chapter  IX).  The 
great  quantities  of  graphite  diffused  through  many  of  the  schists 
and  the  beds  of  iron  ore  likewise  tend  to  show  the  existence  of 
plants  at  the  same  time.  These  indications  do  not  amount  to  a 
proof  of  the  presence  of  life,  for  it  is  possible  that  the  limestone, 
graphite,  and  iron  accumulations  were  made  by  chemical  processes. 
Radiolaria  have  been  reported  from  some  of  the  pre-Cambrian 
schists  of  France  (this  has  lately  been  questioned). 

The  pre-Cambrian  crystalline  rocks  are  remarkable  for  their 
wealth  of  valuable  minerals.  Immense  accumulations  of  iron  ore, 
in  beds  from  TOO  to  400  feet  thick,  occur  in  Canada,  New  York, 
New  Jersey,  along  the  Appalachians  from  Virginia  to  Georgia, 
in  Michigan,  the  Lake  Superior  region,  Missouri,  and  the  South- 
west. The  great  copper  mines  of  Lake  Superior  are  in  igneous 
dikes  which  intersect  sandstones  referred  to  the  Algonkian. 

It  will  be  obvious  to  the  student  how  very  little  is  really  known 
regarding  the  most  ancient  rocks  of  the  earth's  crust.  They  are 
enormously  thick  metamorphic  masses  of  vast  geographical  extent. 
In  all  the  continents  they  form  the  foundation  upon  which  the 
oldest  fossiliferous  sediments  were  laid  down,  and,  in  brief,  they 
are  the  oldest,  the  thickest,  the  most  widely  distributed  and  the 
most  important  of  all  the  accessible  constituents  of  the  earth's  crust. 
Their  uniform  character,  vyherever  found,  the  extreme  plication  and 
metamorphism  which  they  have  undergone,  and  their  world-wide 
distribution,  are  all  extremely  remarkable  features,  such  as  recur 
in  rocks  of  no  other  age.  The  Algonkian  sedimentary  and  meta- 
morphic rocks  seem  to  represent  the  first  series  of  deposits  made 
under  water  and  the  earliest  chapters  in  the  history  of  life.  The 
pre-Cambrian  rocks  indicate  that  vast  periods  of  time  had  elapsed 
before  the  clearly  recorded  part  of  the  earth's  history  began,  a  time 
probably  longer  than  all  subsequent  periods  taken  together. 


CHAPTER   XXII 
THE  PALAEOZOIC  PERIODS  —  CAMBRIAN 

THE  Palaeozoic  is  the  oldest  of  the  three  main  groups  into 
which  the  normal  fossiliferous  strata  are  divided ;  it  forms  the 
first  legible  volume  of  the  earth's  history,  and  in  interpreting  it 
speculation  and  hypothesis  play  a  much  less  prominent  part  than 
in  the  pre-Cambrian  volume.  The  Palaeozoic  rocks  are  con- 
glomerates, sandstones,  shales,  and  limestones,  with  quite  exten- 
sive areas  of  metamorphic  rocks,  and  associated  igneous  masses, 
both  volcanic  and  plutonic.  The  thickness  of  these  rocks  is  very 
great,  estimated  in  Europe  at  a  maximum  of  100,000.  feet.  This 
does  not  imply  that  such  a  thickness  is  found  in  any  one  place, 
but  that  if  th2  maximum  thicknesses  of  each  of  the  subordinate 
divisions  be  added  together,  they  will  amount  to  that  sum.  In 
this  country  more  than  30,000  feet  of  Palaeozoic  strata  are  ex- 
posed in  the  much  folded  and  profoundly  denuded  Appalachian 
Mountains,  but  in  the  Mississippi  valley  they  attain  only  a  fraction 
of  that  thickness.  These  rocks  are,  in  the  vast  majority  of  cases, 
of  marine  origin,  but  some  fresh-water  beds  are  known,  and 
very  extensive  swamp  deposits  have  preserved  a  record  of  much 
of  the  land  life  of  the  era,  especially  of  its  later  portions.  That 
there  must  have  been  land  surfaces  is  abundantly  shown  by  the 
immense  thickness  and  extent  of  the  strata,  all  of  which  were 
derived  from  the  waste  of  the  land.  Both  in  Europe  and  in  North 
America  the  land  areas  were  prevailingly  toward  the  north  and  are 
doubtless  indicated,  in  part,  by  the  great  regions  of  the  pre-Cam- 
brian metamorphic  rocks.  The  general  character  of  the  Palaeo- 
zoic beds  shows  that  they  were,  in  large  measure,  laid  down  in 
shallow  water  in  the  neighbourhood  of  land.  Their  great  thickness 
indicates,  further,  the  enormous  denudation  which  the  land  areas. 

365 


366  PALEOZOIC   PERIODS 

underwent.  The  calculation  has  not  been  made  for  this  country, 
but  for  Great  Britain  Geikie  states  that  the  lower  half  of  the 
Palaeozoic  group  represents  the  waste  of  a  plateau  cut  down  to 
base-level,  larger  than  Spain  and  5000  feet  high. 

Very  wide-spread  disturbances  of  the  earth's  crust  before  the 
beginning  of  the  Palaeozoic  era  and  at  its  close  have  produced 
well-nigh  universal  unconformities  with  both  the  underlying  pre- 
Cambrian  and  the  overlying  Mesozoic  rocks ;  at  only  a  few  points 
are  transitional  series  found. 

Very  early  in  Palaeozoic  time  were  established  the  main  geo- 
graphical outlines  which  dominated  the  growth  of  the  North 
American  continent,  —  a  growth  which  was,  for  the  most  part, 
steady  and  tranquil.  These  conditions  may  be  briefly  stated  as 
the  formation  of  a  great  interior  continental  sea,  divided  from  the 
Atlantic  and  the  Pacific  by  more  or  less  extensive  and  variable 
land  areas.  There  are  thus  three  principal  regions  of  continental 
development :  those  of  the  Atlantic  and  Pacific  borders  and  the 
interior.  In  addition,  the  eastern  border  is  subdivided  by  pre- 
Cambrian  ridges  into  subordinate  areas  of  deposition.  At  the 
present  time  the  surface  rocks  over  the  eastern  half  of  the  conti- 
nent are  prevailingly  Palaeozoic,  extending  chiefly  southward  and 
southeastward  from  the  great  pre-Cambrian  mass  of  the  north. 

Palaeozoic  time  was  of  vast  length,  probably  exceeding  that  of 
the  combined  Mesozoic  and  Cenozoic  eras. 

The  subdivisions  of  the  Palaeozoic  are  very  clearly  marked, 
locally  often  by  unconformities,  but  on  a  wide  scale  by  the 
changes  in  the  character  of  the  fossils.  There  is  some  difference 
in  the  practice  concerning  these  divisions,  not  as  to  their  limits 
or  order  of  succession,  but  merely  as  to  their  rank,  whether  cer- 
tain ones  should  be  called  systems  (periods)  or  series  (epochs). 
This  is  a  difference  more  about  names  than  facts.  The  succes- 
sive steps  of  organic  and  geographical  development  are  best  dis- 
played by  dividing  the  group  into  six  systems,  or  periods,  which 
are  as  follows,  beginning  with  the  oldest:  i.  Cambrian;  2.  Or- 
dovician;  3.  Silurian;  4.  Devonian;  5.  Carboniferous;  6.  Per- 
mian. By  many  geologists  the  Ordovician  and  Silurian  are 


LIFE  367 

comprised  in  one  system,  and  the  Carboniferous  and  Permian  in 
another ;  but  the  present  tendency  is  in  favour  of  maintaining  all 
six  as  equal  in  rank.  It  must  not  be  supposed  that  these  sys- 
tems represent  equal  spaces  of  time  as  measured  by  the  thickness 
of  rocks,  or  equal  geographical  extent ;  on  the  contrary,  they  are 
very  unequal  in  both  these  respects.  The  classification  means 
that  the  six  systems,  or  periods,  stand  for  approximately  equiva- 
lent changes  in  the  character  of  the  animals  and  plants. 

Palaeozoic  Life  possesses  an  individuality  not  less  distinctly 
marked  than  that  of  the  group  of  strata,  which  demarcates  it 
very  sharply  from  the  life  of  succeeding  periods,  and  gives  a  cer- 
tain unity  of  character  to  the  successive  assemblages  of  plants 
(floras}  and  of  animals  (faunas}.  *The  era  is  remarkable  both 
for  what  it  possesses  and  what  it  lacks. ;  Among  plants,  the  vege- 
tation is  made  up  principally  of  Cryptogams,  seaweeds,  ferns, 
club-mosses,  and  horsetails.  Especially  characteristic  are  the 
gigantic,  tree-like  club- mosses  and  horsetails,  which  are  now 
represented  only  by  very  small,  herbaceous  forms.  The  only 
flowering  plants  known  are  the  GyrmTosperms,  the  Cycads  and 
their  allies ;  no  Angiosperms  have  be£n  discovered.  'Palaeozoic 
forests  must  have  been  singularly  gloomy  and  monotonous,  lack- 
ing entirely  the  bright  flowers  and  changing  foliage  of  later  periods/ 

The  Palaeozoic  fauna  is  largely  made  up  of  marine  inverte- 
brates, in  the  earlier  periods  entirely  so,  i.e.  so  far  as  we  have  yet 
learned,  though  land  life  surely  began  before  the  oldest  records 
of  it  yet  discovered.  Corals,  Echinoderms  (especially  Crinoids, 
Cystideans,  and  Blastoids),  Brachiopods,  Mollusca  (particularly 
the  Nautiloid  Cephalopods),  and  the  crustacean  group  of  Trilo- 
bites  are  the  most  abundant  and  characteristic  types  of  animal 
life.  Insects,  centipedes,  and  spiders  were  common  toward  the 
end  of  the  era.  Cambrian  rocks  contain  no  fossil  vertebrates,  but 
they  make  their  appearance  in  the  Silurian,  and  perhaps  earlier. 
For  long  ages  the  only  vertebrates  were. fishes  and  certain  low 
types  allied  to  the  fishes,  but  at  the  end  of  the  Devonian  and  in 
the  Carboniferous  appeared  the  Amphibia,  followed  in  the  Permian 
by  true  Reptiles.  Teleosts,  such  as  make  up  by  far  the  largest 


368  THE  CAMBRIAN   PERIOD 

part  of  the  modern  fish-fauna,  both  marine  and  fresh-water,  as 
well  as  birds  and  mammals,  are  entirely  absent  from  the  Palaeozoic. 

The  overwhelming  majority  of  Palaeozoic  species,  and  even  gen- 
era, fail  to  pass  over  into  the  Mesozoic,  and  even  in  the  larger 
groups  which  continued  to  flourish,  almost  always  a  more  or  less 
complete  change  of  structure  occurs,  so  that  Palaeozoic  corals,  Echi- 
noderms,  and  fishes,  for  example,  are  very  markedly  distinct  from 
those  which  succeeded  them.  The  difference  is  generally  in  the 
direction  of  greater  primitiveness  of  structure  in  the  older  forms, 
Palaeozoic  types  standing  in  somewhat  the  same  relation  to  subse- 
quent types  as  the  embryo  does  to  the  adult. 

The  Palaeozoic  climate  appears  to  have  been  mild  and  equable 
on  the  whole,  very  much  the  same  kinds  of  animals  and  plants  occur- 
ring in  high  as  in  low  latitudes.  In  short,  we  can  detect  no  evi- 
dence of  climatic  zones  as  being  distinctly  marked  in  those  periods. 
Certain  remarkable  exceptions  to  this  rule  will  be  noted  in  their 
proper  place. 

THE  CAMBRIAN  PERIOD  V 

The  rocks  older  than  the  coal  measures  were  for  a  long  time 
V  heaped  indiscriminately  together,  under  the  name  of  Greywacke, 
or  Transition  Rocks,  and  were  little  regarded  by  geologists.  About 
1831,  the  problem  of  these  ancient  rocks  was  attacked  by  two 
eminent  English  geologists,  Sedgwick  and  Murchison,  who  soon 
brought  order  out  of  the  chaos.  There  was  much  discussion  and 
dispute  as  to  the  limits  of  the  systems  into  which  the  Greywacke 
should  be  divided,  and  as  to  the  names  which  should  be  given  to 
them.  The  oldest  fossiliferous  strata  were  by  Sedgwick  called 
Cambrian  (from  the  Latin  name  for  Wales),  but  were  included 
by  Murchison  in  his  Lower  Silurian.  The  latter  example  was  long 
followed  by  most  geologists,  but  the  advance  of  knowledge  has  fully 
vindicated  the  claim  of  the  Cambrian  to  rank  as  a  distinct  system. 
The  divisions  of  the  American  Cambrian  are  as  follows  :  — 

3.   Upper  Cambrian,  Potsdam  Epoch,  Olenus  Fauna. 

2.    Middle  Cambrian,  Acadian  Epoch,  Paradoxides  Fauna. 

i.    Lower  Cambrian,  Georgian  Epoch,  Olenellus  Fauna. 


DISTRIBUTION   OF  CAMBRIAN   ROCKS  369 

American.  —  In  North  America,  Cambrian  rocks  are  not  exposed 
at  the  surface  over  very  large  areas,  being,  for  the  most  part,  deeply 
buried  under  later  sediments.  Their  maximum  thickness,  so  far  as 
known,  does  not  exceed  12,000  feet,  but  this  may  be  considerably 
increased  at  the  expense  of  the  Algonkian.  While  not  forming 
extensive  areas  of  the  present  surface,  Cambrian  strata  are  very 
widely  distributed  over  the  continent,  usually  resting  unconform- 
ably  upon  the  plicated  and  metamorphosed  rocks  of  the  Archaean 
and  Algonkian.  These  strata  are  found  in  the  pre-Cambrian  de- 
pressions, from  the  Adirondacks  to  Newfoundland,  and  along  the 
flanks  of  the  Appalachian  uplift,  from  the  St.  Lawrence  to  Ala- 
bama. They  also  fringe  Archaean  or  Algonkian  areas  in  other 
regions,  as  in  Wisconsin,  Missouri,  Texas,  in  the  Rocky  Mountain 
chain,  from  Colorado  to  British  Columbia,  and  in  the  mountains 
of  Nevada.  Cambrian  beds  are  exposed  in  the  Colorado  Canon, 
and  doubtless  would  be  found  throughout  the  larger  part  of  the 
continent,  were  the  overlying  beds  stripped  away. 

So  far  as  they  are  accessible  to  observation,  the  Cambrian  rocks 
are  chiefly  such  as  are  laid  down  in  shallow  water  near  shore,  con- 
glomerates, sandstones,  shales  (with  some  limestones),  which  are 
ripple-marked  in  a  way  that  betrays  their  shoal-water  origin. 
There  are  also  some  areas  of  deeper  water  accumulations,  found  in 
the  limestones  of  western  Vermont,  Nevada,  and  British  Columbia. 

During  Cambrian  times  the  sea  was  slowly  advancing  over  the 
land  in  North  America,  and  the  geography  of  the  continent  was 
very  different  at  the  close  of  the  period  from  what  it  had  been  at 
the  beginning.  In  the  Lower  Cambrian  the  land  areas  are  inferred 
to  have  been  somewhat  as  follows  :  First,"  there  was  the  great 
northern  .mass  of  crystalline  Archaean  and  Algonkian  rocks,  but 
this  was  probably  much  more  extensive  than  the  present  expos- 
ures of  pre-Cambrian  rocks  would  indicate.  It  probably  covered 
the  whole  Mississippi  valley  down  to  latitude  30°  N.  and  extended 
westward  beyond  the  Rocky  Mountains.  Long,  narrow  strips 
of  land,  alternating  with  narrow  sounds,  occupied  part  of  New 
England  and  the  maritime  provinces  of  Canada,  while  an  Appa- 
lachian land,  whose  western  line  is  marked  by  the  present  Blue 
2  B 


370  THE  CAMBRIAN   PERIOD 

Ridge,  extended  eastward  an  unknown  distance  into  the  Atlantic. 
On  the  western  shore  of  the  Appalachian  land  was  a  narrow  arm 
of  the  sea,  which  opened  both  to  the  north  and  south  and  sepa- 
rated this  land  area  from  the  great  mass  of  the  continent.  The 
site  of  the  Sierra  Nevada  was  occupied  by  a  long,  narrow  land, 
running  from  Puget  Sound  to  Mexico,  and  another  such  area  was 
found  in  eastern  British  Columbia.  The  Great  Basin  region  was 
under  water.  Around  these  shores  were  laid  down  the  coarser 
deposits  of  the  Lower  Cambrian,  with  great  masses  of  shales  and 
some  limestone  in  deeper  water. 

In  the  course  of  time  the  continent  was  slowly  depressed,  the  sea 
gradually  advancing  from  the  south  during  the  Middle  Cambrian, 
and  reaching  its  greatest  extension  in  the  Upper.  Toward  the 
close  of  the  period  a  large  part  of  the  continent  had  been  sub- 
merged and,  in  particular,  a  vast  interior  sea  had  been  established 
over  the  Mississippi  valley. 

Cambrian  in  Other  Continents.  —  In  Europe  the  Cambrian 
rocks  are  even  more  fully  developed  than  in  North  America, 
having  in  Wales  a  thickness  of  20,000  feet  of  conglomerates,  sand- 
stones, shales,  slates,  and  quartzites.  These  rocks  bear  witness  to 
their  shallow-water  origin  and  were  deposited  on  a  slowly  sinking 
sea-bottom.  The  Cambrian  rocks  have  their  maximum  thickness 
along  the  western  side  of  the  continent,  being  four  times  as  thick 
in  Wales  and  Spain  as  in  Germany  and  Bohemia.  In  Russia  the 
rocks  of  this  period  are  remarkable  for  their  unconsolidated 
character ;  at  the  base  are  300  feet  of  plastic  clays,  which  look 
as  though  they  had  just  been  abandoned  by  the  sea.  In  central 
Russia  the  Cambrian  dies  out  and  the  Ordovician  strata  rest 
directly  upon  the  Archaean.  Cambrian  rocks  occur  extensively 
in  northeastern  China,  in  India,  in  Australia,  and  in  the  Argentine 
Republic. 

CAMBRIAN  LIFE 


The  Cambrian  fauna  is  of  extraordinary  interest,  because  it  is 
the  most  ancient  that  we  know,  but  the  most  superficial  examina- 
tion of  it  shows  that  it  cannot  represent  the  beginnings  of  life 


LIFE 


371 


upon  our  planet.  Almost  all  the  great  types  of  invertebrates  are 
already  present  and  very  definitely  characterized,  indicating  that 
life  had  been  differentiating  for  a  vast  period  before  the  lowest 
Cambrian  rocks  had  been  laid  down.  As  compared  with  the 
faunas  of  other  Palaeozoic  periods,  that  of  the  Cambrian  is  very 
scanty,  but  our  knowledge  of  it  has  been  greatly  increased  of 
late  and  may  be  expected  to  increase  in  the  future. 

Though  the  successive  Cambrian  faunas  have  a  very  uniform 
distribution  over  wide  areas,  there  are  already  indications  of  local 
differences  which  mark  out  faunal  provinces;  thus,  the  Middle 
Cambrian  fossils  of  Newfoundland  are  more  similar  to  those  of 
Europe  than  to  those  of  the  Appalachian  and  interior  regions  of 
America.  The  same  fauna  recurs  in  Alabama,  but  not  further 
north  in  the  Appalachians.  The  advance  of  the  sea  gave  to  Upper 
Cambrian  life  a  wider  and  more  uniform  distribution  over  the 
continent  than  to  that  of  the  Lower. 

Of  Plants,  nothing  is  surely  known ;  certain  marks  on  the 
bedding  planes  of  strata  have  been  regarded  as  seaweeds,  but 
they  are  too  obscure  for  determination. 

The  faii^a  is  principa|ly  mari*  up  nf  Brarfrjnppds  and  Trilobites. 
but  many  other  types  are  .represented  also. 

Spongida.  —  Siliceous  Sponges  are  not  uncommon. 

Coelenterata.  —  The  Hydrozoa  are  believed  to  be  represented 
by  the  Graptolites,  a  series  of  forms  which  are  confined  to  the 
older  Palaeozoic  rocks.  These  curious  animals  formed  compound 
colonies,  with  cells  for  the  different  individuals  arranged  on  one 
or  more  sides  of  a  stem,  and  of  a  great  variety  of  form ;  some 
are  straight,  others  spiral,  and  though  commonly  found  in  single 
branches,  some  specimens  have  many  branches  united.  (See 
PL  II,  Fig.  3,  p.  383.)  The  skeleton  was  horny,  and  so  the  fossils 
appear  as  mere  markings  on  the  rocks,  but  often  in  excellent  pres- 
ervation. The  systematic  position  of  the  Graptolites  is  entirely 
uncertain,  though  they  are  usually  referred  to  the  Hydrozoa. 

Other  Hydrozoa  are  the  jelly-fish,  of  which  recognizable  casts 
have  been  found  in  large  numbers. 

It  is  still  a  question  whether  Corals  were  present  in  the  Cam- 


3/2  THE  CAMBRIAN   PERIOD 

brian ;  certain  fossils  which  by  some  authorities  are  called  corals 
are  by  others  regarded  as  sponges.  At  all  events,  they  are  not 
conspicuous  elements  in  the  fauna. 

Echinodermata.  —  The  Echinoderms  are  rare  and  are  prin- 
cipally Cystids,  a  very  primitive  grade  of  the  type ;  true  Crinoids 
and  Star-fishes  appear  before  the  close  of  the  period. 

Worms.  —  The  presence  of  marine  worms  is  indicated  by  tracks 
and  borings  in  the  sands  which  have  now  consolidated  into  hard 
rocks. 

Arthropoda.  —  The  only  known  Cambrian  Arthropods  are  the 
Crustacea,  and  of  these  much  the  most  abundant  group  is  that  of 
the  Trilobita,  which  are  altogether  confined  to  the  Palaeozoic 
rocks  and  are  by  far  the  most  important  of  Cambrian  fossils.  It 
is  only  within  recent  years  that  the  systematic  position  of  the 
Trilobites  has  been  established  through  the  fortunate  discovery 
of  specimens  with  their  appendages  attached  (see  Fig.  138).  Tri- 
lobites have  a  more  or  less  distinctly  three-lobed  body,  at  one  end 
of  which  is  the  head-shield,  usually  with  a  pair  of  fixed  compound 
eyes ;  at  the  other  end  is  the  tail-shield,  and  between  the  two 
shields  is  a  ringed  or  jointed  body  made  up  of  a  variable  number 
of  movable  segments.  The  Trilobites  display  an  extraordinary 
variety  in  form  and  size,  in  the  proportions  of  the  head  and  tail- 
shields,  in  the  number  of  free  segments,  and  in  the  development  of 
spines.  Already  in  the  Cambrian  this  wealth  of  forms  is  notable, 
though  far  less  so  than  it  became  in  the  Ordovician.  As  compared 
with  those  of  later  times,  the  Cambrian  Trilobites  are  marked  by 
the  (usually)  very  small  size  of  the  tail-shield,  the  large  number  of 
free  segments,  and  their  inability  to  roll  themselves  up.  Some  of 
them,  like  Paradoxides,  are  very  large  (from  10  inches  to  2  feet 
in  length).  Olenellus  also  has  large  species,  while  Agnostiis  is  ex- 
cessively small  and  without  eyes. 

The  great  importance  of  the  Trilobites  for  Cambrian  stratigraphy 
is  indicated  by  the  fact  that  the  three  divisions  of  the  system 
are  named  for  the  three  dominant  genera  of  these  crustaceans, 
Olenellus,  Paradoxides,  and  Olenus. 

Two  other  divisions  of  the  Crustacea  are  found  in  the  Cam- 


BRACH1OPODA 


373 


brian :  the  Osfracoda,  little  bivalve  forms,  whose  shells  look 
deceptively  like  those  of  molluscs;  and  the  Phyllopoda,  which 
have  a  large  shield  on  the  head  and  thorax,  and  a  many-jointed 
abdomen. 


PLATE  I.   AMERICAN  CAMBRIAN  FOSSILS 


i.  Lingulellacoelata,2/i.  (Walcott.)  2.  Agnostus  interstrictus.a/i.  (Walcott.) 
3.  Conocoryphe  Kingi.  (Meek.)  4.  Elliptocephalus  Thompsoni.  (Walcott.) 
5.  Olenoides  typicalis.  (Walcott.) 

Brachiopoda.  —  These  are  among  the  most  abundant  of  Cam- 
brian fossils ;  most  of  them  belong  to  the  lower  order  of  the  class 
{iMrtfculaia),  in  which  the  shells  are  mostly  horny  and  the  two 
valves  are  not  articulated  together  bv^a^toge.  The  horny-shelled 
Discina  and  Lingulella  are  of  great  interest,  for  they  have  per- 
sisted through  all  the  changes  and  vicissitudes  of  the  earth  down 
to  the  present  day,  with  hardly  any  modification.  The  second 


3/4  THE   CAMBRIAN   PERIOD 

order  of  Brachiopods,  the  Articulata,  which  have  calcareous  shells 
connected  by  an  elaborate  hinge,  became  more  common  in  the 
Upper  Cambrian.  They  soon  grow  vastly  more  numerous  than 
the  Inarticulata,  and  throughout  the  post-Cambrian  divisions  of 
the  Palaeozoic  their  shells  are  found  in  incalculable  numbers. 

The  Mollusca  are  already  represented  by  their  principal  divi- 
sions. The  Pelycypoda,  or  Bivalves,  are  of  very  small  size  and 
found  very  scantily ;  their  variety  and  relative  importance  have 
gone  on  increasing  ever  since  Cambrian  times.  Gastropoda  occur 
in  small  numbers,  especially  in  the  Upper  Cambrian.  Fossils  re- 
ferred, with  some  doubt,  to  the  Pteropoda  are  among  the  most 
frequent  of  shells  found  in  these  rocks,  but  display  no  great 
variety.  The  Cephalopoda,  which  are  the  highest  group  of  mol- 
luscs, are  at  present  represented  by  two  suborders ;  in  one,  the 
squids  and  cuttle-fishes  {Dibranchiata) ,  the  shell  is  rudimentary 
and  internal;  while  in  the  other  {Tetrabranchiata}  the  shell  is 
external.  The  latter  kind  of  shell  is  divided  by  transverse  septa 
into  chambers,  which  are  connected  by  means  of  a  tube,  the 
siphuncle,  the  animal  living  only  in  the  terminal  chamber  at 
the  mouth  of  the  shell.  The  only  living  representative  of  this 
group  is  the  pearly  Nautilus,  but  throughout  Mesozoic  and 
Palaeozoic  time  there  was  a  great  variety  of  these  chambered 
shells.  In  the  Cambrian  the  Cephalopods  are  very  few  and 
almost  confined  to  the  uppermost  part  of  the  system. 

The  Cambrian  fauna  displays  steady  progress,  being  distinctly 
more  advanced  in  the  upper  than  in  the  lower  division.  The 
Middle  Cambrian  fauna,  so  far  as  it  is  yet  known,  has  not  nearly 
so  many  species  as  the  Lower,  but  this  is  doubtless  due  to  unfa- 
vourable conditions  of  preservation. 


CHAPTER    XXIII 
ORDOVICIAN   (OR  LOWER  SILURIAN)   PERIOD 

MURCHISON  divided  his  great  Silurian  system  primarily  into 
two  parts,  Upper  and  Lower.  This  method  of  classification  is 
generally  followed  even  at  the  present  day,  although  it  is  widely 
recognized  that  the  most  decided  break  in  the  entire  Palaeozoic 
group  is  the  one  between  these  divisions.  In  1879  Lapworth 
proposed  to  give  due  emphasis  to  this  distinction  by  erecting 
the  Lower  Silurian  into  a  separate  system,  the  Ordovidan.  The 
name  is  taken  from  the  Ordovici,  an  ancient  British  tribe  which 
dwelt  in  Wales  during  Roman  times.  Lapworth's  example  is 
now  largely  followed  in  England  and  the  United  States,  but  on 
the  continent  of  Europe  the  name  Silurian  is  still  retained  for 
both  systems. 

The  classification  and  subdivision  of  the  American  Ordovician 
were  first  worked  out  in  the  state  of  New  York,  and  consequently 
the  New  York  scale  serves  as  a  standard  of  reference  for  the  rest 
of  the  continent.  It  is  given  in  the  following  table  from  Dana  :  — 

3.  Hudson  or  Cincinnati  Stage. 
2.  Utica  Stage. 


I.  Trenton  Stage. 

f  2.  Chazy  Stage. 
I.  Canadian  Series.  ]  .; 

(  i.  Calciferous  Stage. 


The  passage  from  the  Cambrian  to  the  Ordovician  was  a  grad- 
ual one,  not  marked  either  in  North  America  or  in  Europe  by  any 
decided  physical  break,  but  by  a  change  in  the  character  of  the 
fossils.  By  the  end  of  the  Cambrian  period  a  vast  interior  sea 
had  been  established  over  what  is  now  the  Mississippi  valley. 
This  great  sea  was  separated  from  the  Atlantic  by  the  Appalachian 

375 


376  THE  ORDOVICIAN   PERIOD 

land,  and  on  the  west  islands  of  unknown  size  demarcated  it 
from  the  Pacific.  The  Ordovician  rocks  accumulated  in  the  Inte- 
rior Sea  were  principally  limestones  and  dolomites,  while  at  the 
east  extensive  sandstones  and  slates  were  formed  at  the  begin- 
ning and  toward  the  end  of  the  period.  Ordovician  rocks  have  a 
much  wider  extension  than  would  appear  from  their  surface  dis- 
tribution, for  they  are  generally  buried  under  sediments  of  a  later 
date.  In  the  west,  for  example,  they  are  exposed  at  the  bottom 
of  many  deep  canons,  lying  beneath  thousands  of  feet  of  younger 
beds.  In  the  southwest  of  the  United  States  was  a  land  area  of 
unknown  extent.  In  the  Grand  Canon  section  no  strata  occur 
between  the  Cambrian  and  the  Carboniferous,  while  from  Mexico 
no  rocks  older  than  the  Carboniferous  have  been  reported. 

Around  the  northern  pre-Cambrian  land,  in  New  York  and 
Canada,  was  formed  the  Calciferous,  a  limestone,  generally  magne- 
sian  and  often  sandy  or  cherty,  which  extends  southward  through 
New  Jersey  and  Pennsylvania,  while  equivalents  of  it  are  found  in 
the  magnesian  limestones  of  Iowa,  Missouri,  and  Michigan. 

Deepening  waters  next  gave  opportunity  for  the  formation  of 
limestones  (Chazy)  which  grew  to  a  vast  extension,  especially  the 
great  formation  called  the  Trenton,  which  is  developed  in  New 
Brunswick,  New  York,  Canada,  the  upper  Mississippi  valley,  and 
in  the  Rocky  Mountains.  Toward  the  end  of  the  Ordovician 
period  there  was  a  change  in  the  eastern  part  of  the  Interior  Sea, 
whereby  the  clear  waters  became  charged  with  fine  mud  and  clay, 
which  now  form  a  great  mass  of  shales  and  slates  (Ufica  and 
Hudson).  These  rocks  are  thickest  toward  the  east,  extending 
from  the  St.  Lawrence  along  the  Appalachian  uplift  into  east 
Tennessee,  where  they  become  much  thinner  and  are  in  many 
places  represented  by  shaly  limestones,  and  westward  into  Indiana. 
The  effects  of  the  change  were  very  widely  felt. 

Ordovician  rocks,  prevailingly  limestones,  are  also  extensively 
displayed  in  the  western  mountain  ranges,  the  Rockies,  Uintas, 
Wasatch,  and  others ;  they  fringe  the  western  side  of  the  great 
northern  pre-Cambrian  area  and  recur  in  the  islands  of  the  Arctic 
Ocean. 


CLOSE   OF  ORDOVICIAN  377 

Foreign.  —  In  Europe  the  Ordovician  rocks  appear  to  have  been 
laid  down  in  two  distinct  seas  separated  by  a  ridge  of  land.  The 
northern  area  extends  from  Ireland  far  into  Russia,  while  the 
southern  is  represented  by  numerous  scattered  masses.  These 
rocks  cover  a  much  wider  surface  than  do  the  Cambrian.  In 
Great  Britain,  especially  in  Wales,  they  form  very  thick  masses  of 
shales  and  slates,  with  but  little  limestone,  intercalated  with  much 
volcanic  lava  and  tuff.  In  Scandinavia  these  rocks  are  nearly 
horizontal  limestones  and  shales,  and  in  Russia  they  cover  very 
large  areas  and  are  so  perfectly  undisturbed  that  many  are  still 
incoherent  sediments.  In  the  southern  sea  were  laid  down  the 
Ordovician  strata  of  Bohemia,  Germany,  northwestern  and  central 
France,  Spain,  Portugal,  Sardinia,  and  Morocco. 

The  very  marked  difference  between  the  fossils  of  the  two  great 
European  areas,  and  the  fact  that  the  Ordovician  fossils  of  other 
continents  agree  with  those  of  northern  Europe,  while  those  of 
the  southern  district  are  peculiar,  indicate  that  the  latter  region 
was  a  partially  closed  sea,  which  occupied  the  Mediterranean 
basin,  though  extending  far  beyond  its  present  limits. 

Asia  appears  to  have  been  principally  dry  land  during  the  Ordo- 
vician, but  with  a  broad  Indo-Chinese  sea  covering  its  eastern 
shore.  Africa  has  Ordovician  rocks  only  in  the  Mediterranean 
region ;  in  the  southern  part  of  the  continent  nothing  older  than 
Devonian  is  known.  The  Australian  continent  of  Cambrian  times 
was  partially  submerged  in  the  Ordovician,  and  strata  of  this  date 
were  laid  down  in  New  Zealand,  Tasmania,  and  the  southern  part 
of  Australia.  But  little  of  South  America  was  submerged  and  that 
continent  may  have  been  considerably  larger  than  at  present. 
Rocks  of  this  system  are  known  only  from  Bolivia  and  Argentina. 

Close  of  Ordovician. — The  period  was  a  time  of  tranquil  depo- 
sition of  sediments,  with  some  oscillations  of  level  and  changes  in 
the  depth  of  water  and  position  of  the  shore  line,  as  indicated  by 
the  alternations  of  the  strata.  At  the  close  of  the  period  came 
a  time  of  wide-spread  disturbance,  upheaval,  and  mountain  mak- 
ing, the  traces  of  which  may  still  be  plainly  observed  in  North 
America  and  Europe,  especially  along  the  Atlantic  slope  of  each 


378  THE  ORDOV1CIAN   PERIOD 

continent.  In  Nova  Scotia  and  New  Brunswick  the  Silurian 
strata  lie  unconformably  upon  the  upturned  Ordovician.  Along 
the  line  between  New  York  and  New  England  the  Taconic  range 
was  upheaved,  its  rocks  greatly  compressed,  plicated,  faulted,  and 
metamorphosed.  Many  of  the  crystalline  schists  of  this  region,  it 
has  been  proved,  were  derived  from  the  metamorphosis  of  Cam- 
brian and  Ordovician  sedimentary  rocks.  Evidences  of  this  dis- 
turbance have  been  traced  as  far  south  as  Virginia.  The  effects 
of  the  upheaval  were  not  felt  in  the  northern  part  of  the  Gulf  of 
St.  Lawrence,  for  on  Anticosti  Island  the  great  limestone,  which 
was  begun  in  Ordovician  times,  continued  without  a  break  into 
the  Silurian.  The  disturbance  was  along  a  line  of  especially  thick 
accumulations,  as  appears  from  the  comparative  measurements  of 
the  same  strata  in  different  areas.  Westward  over  the  Interior 
Sea,  the  upheaval  was,  for  the  most  part,  of  slight  amount,  so  that 
in  this  region  there  are  no  very  marked  unconformities  between 
the  Ordovician  and  the  overlying  Silurian.  Some  narrow  strips 
of  land  were  added  to  the  margin  of  the  Cambrian  coasts,  and  on 
a  line  running  through  southern  Ohio,  Kentucky,  and  Tennessee 
a  low,  broad  arch  was  forced  up  by  lateral  compression.  This 
is  called  the  "  Cincinnati  anticline." 

In  Europe  the  disturbances  which  brought  the  Ordovician  to  a 
close  produced  their  maximum  effects  in  England,  Wales,  and  the 
Highlands  of  Scotland,  where  the  thickness  of  the  sediments  is 
greatest.  In  these  regions  the  Ordovician  beds  are  folded  and 
often  greatly  metamorphosed,  the  Silurian  strata  lying  upon  their 
upturned  edges. 

THE  LIFE  OF  THE  ORDOVICIAN 

Ordovician  life  displays  a  notable  advance  over  that  of  the 
Cambrian,  becoming  not  only  very  much  more  varied  and  luxu- 
riant, but  also  of  a  distinctly  higher  grade.  During  the  long  ages 
of  the  period  also  very  decided  progress  was  made,  and  when 
the  Ordovician  came  to  its  close,  all  of  the  great  types  of  marine 
invertebrates  and  most  of  their  more  important  subdivisions  had 
come  into  existence.  In  a  general  way  the  life  of  the  Ordovician 


CCELENTERATA  379 

is  an  expansion  of  that  of  the  Cambrian,  though  but  little  direct 
connection  between  the  two  can  yet  be  traced,  and  evidently  there 
were  great  migrations  of  marine  animals  from  some  region  which 
cannot  yet  be  identified.     Several  groups  of  invertebrates  attained 
their  culmination  and  began  to  decline  in  the  Ordovician,  becom- 
ing much  less  important  in  subsequent  periods.     Thus  the  Grapto- 
lites,  the  Cystidean  order  of  Echinoderms,  the  Pteropods  among   ,  / 
Molluscs,  and  the  Trilobites  were  never  so  abundant  and  so  varied    y 
as  during  this  period. 

Plants.  —  In  America  no  plants  above  the  grade  of  seaweeds 
have  been  discovered,  but  in  Europe  a  few  of  the  higher  Crypto- 
gams are  doubtfully  reported.     The  flora  of  the  Devonian,  how- 
ever, renders  it  highly  probable  that  land  plants  were  already  well 
advanced  in  the  Ordovician,  and  their  remains  may  be  discovered 
f  at  any  time.     This   must  remain  a  matter  of  accident,  for  the 
known  Ordovician  rocks  are  all  marine,  which  is  not  a  favourable 
I    circumstance  for  the  preservation  of  land  plants.     Such  discov- 
\   cries  have,  indeed,  already  been  reported,  but  the  evidence  for 
them  is  not  satisfactory. 

Foraminifera  and  Radiolaria  have  been  found  in  sufficient  num- 
bers to  prove  that  they  were  abundant  in  the  Ordovician  seas. 

Spongida.  —  Sponges  are  much  more  numerous  and  varied  than 
in  the  Cambrian.  Of  course  it  is  only  those  sponges  with  skeletons 
of  lime  or  flint  which  can  be  well  preserved  in  the  fossil  state,  and 
of  these  the  Ordovician  has  many  (PI.  II,  Fig.  i).  The  horny 
sponges,  of  which  the  common  bath  sponge  is  a  familiar  example, 
are  necessarily  much  rarer  and  less  satisfactory  as  fossils. 

Coelenterata.  —  The  Graptolites  are  very  numerous  and  varied, 
wherever  conditions  are  favourable  to  their  preservation,  as  in 
fine-grained  rocks  with  smooth  bedding  planes.  The  Ordovician 
is  the  time  of  their  culmination  and  is  especially  characterized  by 
the  double  forms,  with  rows  of  cells  on  both  sides  of  the  stem 
(see  PI.  II,  Figs.  2,  3,  4).  So  abundant  are  the  Graptolites  that 
in  many  parts  of  the  system  they  are  almost  the  only  fossils  and 
are  employed  to  divide  the  substages  into  zones.  The  few  and 
doubtful  Cambrian  Corals  are  succeeded  by  a  considerable  number 


380 


THE  ORDOVICIAN   PERIOD 


of  Ordovician  genera  and  species.     Like  other  Palaeozoic  Corals, 
these  are  characteristically  different  from  the  reef-builders  of  the 

present  day  in  showing  a  marked 
bilateral  symmetry  and  having  the 
septa  arranged  in  multiples  of  four 
(  Tetracoralla ) .  Large,  solitary  cup- 
corals,  like  Streptelasma,  and  com- 
pound colonies,  like  Favistella  and 
Columnaria,  are  examples  of  the 
range  of  differences  among  these 
early  corals. 

The  Echinodermata  have  greatly 
increased  in  importance,  and  except 
the  Echinoids,  all  the  main  subdivi- 
sions of  the  group  are  represented. 
The  Cystidea,  which  we  have  already 
found  in  the  Cambrian,  attain  their 
greatest  development  in  the  Ordo- 
vician. In  these  curious  animals  the 
body  is  either  irregularly  shaped,  or 
symmetrical,  with  a  short,  tapering 
stem,  by  which  the  animal  was  at- 
tached to  the  sea-floor,  and  weakly  developed  arms.  The  body, 
or  calyx,  is  made  up  of  a  number  of  calcareous  plates ;  when 
these  plates  are  very  numerous,  they  are  of  irregular  size  and 
arrangement  (PI.  II,  Fig.  6),  while  the  forms  with  few  plates  have 
them  of  a  definite  number,  size,  and  shape  (PI.  Ill,  Fig.  i). 
Some  of  the  more  regular  Cystidea  have  much  resemblance  to 
the  true  Crinoids.  The  latter  make  their  first  appearance  in  the 
upper  part  of  the  Cambrian,  but  in  the  Ordovician  they  greatly 
increase  in  numbers  and  importance,  though  less  abundant  than 
they  afterwards  became.  These  animals  (PI.  II,  Fig.  5)  have  a 
symmetrical  calyx,  with  long,  branching  arms ;  the  number  and 
arrangement  of  the  component  plates  are  definite  and  character- 
istic for  each  genus.  Most,  but  not  all,  of  the  Crinoids  have  a 
long,  jointed  stem,  by  which  they  were  attached  to  the  sea-bottom, 


FIG.  137.  —  Ordovician  Coral, 
Favistella  stellata.  Hudson  Stage, 
New  York.  (Hall.) 


TRILOBITES 


381 


Asteroids  (star-fishes)  and  Ophiuroids  (brittle-stars),  which  had 
likewise  come  in  before  the  end  of  the  Cambrian,  increase 
in  the  Ordovician.  A 
new  order  of  Echino- 
derms,  the  Echinoidea, 
or  Sea-urchins,  first  ap- 
pear in  the  Ordovician, 
being  represented  by  very 
primitive  forms. 

Arthropod  a.  --  The 
Trilobites  increase  very 
greatly  in  the  number  of 
genera  and  species,  and 
most  of  the  Cambrian 
genera  are  replaced  by 
new  ones.  This  is  the 
period  in  which  the  group 
of  Trilobites  attains  its 
highest  development, 
gradually  declining  after- 
ward and  becoming  extinct 
with  the  close  of  the  Palae- 
ozoic. The  most  charac- 
teristic and  widely  spread 
genera  of  Ordovician  Tri- 
lobites are  :  Asaphus  (PL 
II,  Fig.  13),  Ill&nus,  Tri- 
arthrus  (PI.  II,  Fig.  15), 


FIG.  138.  — Ordovician  Trilobite.  Triarthrus 
Becki,  enlarged  restoration  showing  appendages. 
(Beecher.)  Utica  Stage,  New  York. 


Calymene  (II,  14),  Trinucleus  (II,  16),  Dalrnanites,  etc.  These 
genera  differ  in  aspect  from  those  of  the  Cambrian  in  their  much 
larger  tail-shields,  in  their  ability  to  roll  themselves  up  (see  II, 
14),  and  in  their  larger  and  better  developed,  faceted  eyes. 

Other  Crustacea  mark  great  advances  in  the  Ordovician.  Thus, 
in  the  upper  part  of  the  system  we  find  the  first  of  the  Cirripedia, 
or  Barnacles,  a  degenerate,  sedentary  type,  and  the  first  of  the 
Euryfterida,)  a  group  which  is  destined  to  a  remarkable  develop- 


382  THE  ORDOVICIAN    PERIOD 

ment  in  the  Silurian  and  Devonian.  Ostracoda  (PL  II,  Fig.  17) 
and  Phyllopoda  undergo  no  marked  change.  That  terrestrial  ani- 
mal life  had  already  begun  is  demonstrated  by  the  occurrence  of 
a  Centipede. 

Brachiopoda. — These  shells  increase  very  largely  in  abundance 
and  variety,  the  genera  with  hinged  calcareous  shells  (Articulata) 
now  gaining  the  upper  hand  and  reducing  the  horny-shelled  kinds 
to  comparative  insignificance.  The  most  important  genera  are  : 
Orthis  (PL  II,  Fig.  7),  Orthisina,  Leptcena,  Strophomena  (PI. 
II,  Fig.  9),  Platystrophia  and  Rhynchonella. 

Bryozoa.  —  This  is  a  group  which  has  as  yet  yielded  no  repre- 
sentatives from  the  Cambrian,  but  appears  in  the  Ordovician.  The 
genera  differ  little  from  those  which  live  in  the  modern  seas. 

Mollusca.  —  One  of  the  most  striking  differences  between  the 
Ordovician  and  the  Cambrian  is  the  great  advance  made  by  the 
Molluscs  in  the  former  period.  The  Bivalves  (Pelycypoda)  are 
larger,  more  numerous,  and  more  like  modern  forms  (see  PI.  II, 
Fig.  10).  The  Gastropoda  likewise  increase  notably  in  size  and 
in  numbers,  especially  the  spirally  coiled  shells  like  Murchisonia 
(PL  II,  Fig.  n)  and  Pleu roto ma ria ;  but  neither  Bivalves  nor 
Gastropods  had  anything  like  the  relative  importance  which  they 
possess  in  modern  times. 

Much  the  most  significant  change  in  the  Mollusca,  however,  is 
the  great  expansion  of  the  Cephalopoda,  a  few  of  which  appear  in 
the  uppermost  Cambrian,  but  in  the  Ordovician  have  become  one 
of  the  predominant  elements  in  the  marine  life  of  the  times. 
These  forms  are  all  Nautiloids,  most  nearly  allied  to  the  modern 
pearly  Nautilus,  with  chambered  shells,  divided  internally  by  simple 


EXPLANATION  OF  PLATE  II,  p.  383.  i.  Brachiospongia  digitata,  1/4.  2.  Di- 
cranograptus  ramosus,  1/2.  (Hall.)  3.  Diplograpttis  pristis.  '  (Ruedemann.) 
4.  Phyllograptus  typus.  (Hall.)  5.  Dendrocrinus  polydactylus,  1/2.  (Meek.) 
6.  Agelacrinus  cincinnatiensis,  2/1.  (Meek.)  7.  Orthis  lynx.  8.  Rhynchonella 
capax.  9.  Strophomena  alternata.  10.  Ambonychiaradiata,  2/3.  (Hall  and  Whit- 
field.)  it.  Murchisonia  Milled,  2/3.  (Hall.)  12.  Orthoceras  Duseri,  1/2.  (Hall 
and  Whitfield.)  13.  Asaphus  gigas,  1/3.  (Hall.)  14.  Calymene  callicephala,  2/3. 
(Meek.)  15.  Triarthrus  Becki,  2/3.  (Hall.)  16.  Trinucleus  concentricus.  (Hall.) 
17.  Leperditia  fabulites.  (Ulrich.) 


ORDOVICIAN  FOSSILS 


383 


PLATE  II.   AMERICAN  ORDOVICIAN  FOSSILS 


384  THE  ORDOVICIAN   PERIOD 

septa.  The  commonest  shell  of  this  type  is  Orthoceras,  which  is 
a  straight  and  very  elongate  cone  (PI.  II,  Fig.  12)  and  sometimes 
attains  a  length  of  10  feet ;  the  genus  persists  throughout  the  Palae- 
ozoic, and  into  the  Mesozoic.  Endoceras,  which  likewise  has  a 
straight  shell,  with  a  curiously  complex  siphuncle,  is  confined  to 
the  Ordovician.  Besides  these  straight  forms  we  find  curved  shells 
like  Cyrtoceras,  shells  like  Lituites,  coiled  at  one  end,  with  a  long, 
straight  terminal  portion,  resembling  an  Orthoceras  with  its  smaller 
end  rolled  up  into  a  coil.  Others  again,  like  Trocholites,  have  the 
shell  coiled  in  a  close,  flat  spiral. 

Vertebrata.  —  The  curious,  mail-clad  Ostracoderms,  primitive 
vertebrates  which  somewhat  resemble  the  fishes  in  appearance, 
have  been  reported  from  Ordovician  sandstones  of  Colorado.  As 
these  remains  are  very  imperfect  and  as  the  geological  position  of 
the  beds  has  been  questioned,  description  of  the  Ostracoderms  will 
be  deferred  till  a  later  chapter.  Teeth  of  true  fishes  have  been 
found  in  the  Ordovician  of  Europe. 


CHAPTER    XXIV 
THE  SILURIAN    (UPPER   SILURIAN)   PERIOD 

THE  name  Silurian,  like  Cambrian  and  Ordovician,  refers  to 
Wales.  The  term  was  proposed  by  Murchison  in  1835  f°r  a  great 
system  of  strata  older  than  the  Devonian,  and  was  taken  from  the 
Silures,  another  ancient  tribe  of  Britons  which  inhabited  part  of 
Wales.  Murchison  gave  great  extension  to  his  Silurian  system, 
including  in  it  most  of  Sedgwick's  Cambrian,  but,  as  already  pointed 
out,  the  present  tendency  is  to  divide  this  vast  succession  of  rocks 
into  three  systems  of  equivalent  rank.  It  is  unfortunate,  and  even 
unjust,  that  Murchison's  term  should  not  have  been  retained  for 
the  more  important  and  widely  developed  lower  division,  now 
called  the  Ordovician,  rather  than  for  the  upper  division. 

As  in  the  Ordovician  and  Devonian,  the  New  York  classification, 
given  in  tabular  form  below,  is  the  standard  of  reference  for  the 
American  Silurian. 


Silurian 
System. 


f  3.  Upper  Pentamerus  Stage. 
3.  Lower  Helderberg     J    c^\    _  . 

&  \  2.  Shaly  Limestone  Stage. 
Series. 

(  3.  Lower  Pentamerus  Stage. 

2.  Onondaga  Series.  Salina  and  Water-lime  beds. 

f  3.  Niagara  Stage. 
I.  Niagara  Series.        -I  2.  Clinton  Stage. 

[  i.  Medina  Stage. 


American. — The  disturbance  which  closed  the  Ordovician  does 
not  appear  to  have  materially  enlarged  the  extent  of  the  con- 
tinent, and  at  the  beginning  of  the  Silurian  the  general  disposition 
of  land  and  water  was  much  what  it  had  been  before.  A  narrow 
strip  of  coast  lands  had  been  added  to  the  shore  of  the  northern 
pre-Cambrian  land  mass,  converting  Minnesota,  Wisconsin,  much 
of  the  province  of  Ontario,  northern  New  York,  and  New  Jersey, 
sc  385 


386  THE   SILURIAN   PERIOD 

and  western  New  England,  into  permanent  land.  The  Appalach- 
ian land  was  enlarged  and  connected  with  the  northern  area,  shut- 
ting off  the  straits  which  had  formerly  separated  the  two  areas  and 
had  joined  the  Interior  Sea  with  the  Gulf  of  St.  Lawrence.  South- 
ern Ohio  and  central  Kentucky  and  Tennessee  had  been  uplifted 
into  one  or  more  considerable  islands,  which  shut  off  the  north- 
eastern portion  of  the  Interior  Sea  as  a  partially  closed  gulf,  while 
another  island  occupied  southern  Missouri.  What  changes  affected 
the  land  masses  of  the  far  West  and  Southwest  cannot  yet  be  defi- 
nitely determined,  but  it  is  probable  that  they  were  enlarged. 

The  Silurian  rocks  are  much  better  developed  and  far  thicker 
in  the  East,  especially  along  the  Appalachian  range,  where  they 
attain  great  thickness,  than  in  the  interior  or  western  regions,  for 
they  thin  out  or  are  wanting  over  large  parts  of  the  latter.  So  far 
as  they  have  been  identified,  these  rocks  were  laid  down  in  the 
Interior  Sea  or  in  the  narrow  channels  which  still  extended  from 
the  Gulf  of  St.  Lawrence  southward  into  New  England. 

The  period  opened  with  the  formation  in  the  East  of  a  thick 
conglomerate  of  quartz  pebbles  and  sand,  the  Oneida  (a  substage 
of  the  Medina).  This  conglomerate  is  extensive  in  central  New 
York,  thinning  toward  the  eastern  shore  line,  but  very  thick  along 
the  Appalachian  ranges  as  far  south  as  Tennessee.  Owing  to  its 
hardness  and  resistant  qualities,  it  forms  the  crest  of  many  of  the 
mountain  ridges,  as  of  the  Kittatinny,  at  the  Delaware  Water 
Gap.  Farther  west  and  overlying  the  conglomerate  is  a  great  mass 
of  sandstone,  the  Medina,  which  was  accumulated  in  very  shallow 
water,  for  it  is  abundantly  ripple-marked.  The  sandstone  extends 
westward  from  central  New  York,  thinning  out  to  a  belt  of  shale 
in  Ohio ;  reduced  to  a  thickness  of  300  to  400  feet  in  Ontario,  it 
does  not  reach  Michigan,  but  is  very  thick  in  eastern  Pennsyl- 
vania (1800  feet).  No  equivalent  of  these  beds  is  known  in  the 
interior. 

In  New  York  and  Pennsylvania  the  sandstone  is  followed  by  a 
series  of  shales  and  shaly  sandstones  with  some  beds  of  limestone, 
the  Clinton,  which  is  much  more  widely  extended,  reaching  south- 
ward along  the  Appalachians  to  Georgia,  and  westward  as  far  as 


AMERICAN  387 

Wisconsin.  The  sea  evidently  deepened  to  the  westward,  for  here 
the  Clinton  is  represented  by  limestones.  Next  came  a  time  of 
limestone  making  on  a  great  scale  (the  Niagara),  indicating  a  gen- 
eral deepening  of  the  water,  even  to  the  eastward.  The  gradual 
nature  of  the  change  is  shown  by  the  fact  that  in  western  New 
York  the  lower  part  of  this  stage  is  a  shale,  with  limestone  above. 
This  arrangement  is  beautifully  displayed  in  the  gorge  of  the 
Niagara  River  (see  p.  100),  and  to  it  is  due  the  continued  verti- 
cality  of  the  falls.  In  the  East  the  Niagara  limestone  has  little 
southward  extension,  not  occurring  even  in  Pennsylvania,  but 
southwestward  it  stretches  for  nearly  1000  miles,  to  Wisconsin  and 
thence  over  Illinois,  Iowa,  Missouri,  and  western  Tennessee.  It 
recurs  in  the  Black  Hills  of  South  Dakota,  and  in  Nevada  is 
represented  by  the  summit  of  a  thick  mass  of  limestone,  which 
extends  upward  unbrokenly  from  the  Trenton.  The  Niagara 
limestone  is  very  largely  made  up  of  corals  and  in  some  places 
the  ancient  reefs  may  be  identified.  In  the  narrow  channels  to 
the  northeastward,  connected  with  the  Gulf  of  St.  Lawrence, 
were  laid  down  the  Niagara  rocks  which  now  are  found  in  New 
England,  New  Brunswick,  and  Nova  Scotia. 

The  next  change  was  the  separation,  along  the  northern  por- 
tion of  the  interior  sea,  of  a  series  of  salt-water  lagoons,  in  which 
were  deposited  red  marls  and  shales  with  gypsum  and  rock  salt 
{Salina},  from  which  are  obtained  the  brines  of  New  York,  On- 
tario, and  Ohio.  These  rocks  are  thickest  in  New  York  and 
Pennsylvania,  thinning  to  the  south  and  west.  In  part  contem- 
poraneous with  the  Salina  is  the  Water-lime,  so  called  because 
it  is  a  hydraulic  limestone  and  employed  for  making  cement.  It 
is  a  thinly  bedded  argillaceous  and  magnesian  limestone,  in  places 
enclosing  masses  of  gypsum.  The  Water- lime  has  much  the  same 
distribution  as  the  Salina,  but  is  thickest  where  the  latter  is  thin. 

A  renewed  change  of  level,  this  time  a  depression,  brought 
the  sea  in  again  where  the  salt  lagoons  had  been,  extending  it 
farther  to  the  eastward  at  the  same  time.  In  this  clearer  and 
deeper  sea  were  laid  down  the  limestones  of  the  Lower  Helderberg 
series.  The  old  channel  between  the  Appalachian  and  the  north- 


388  THE  SILURIAN  PERIOD 

ern  lands  was  probably  reopened  by  this  disturbance,  by  way  of 
the  upper  Hudson  and  Lake  Champlain,  for  near  Montreal  the 
beds  of  this  series  rest  directly  upon  Ordovician  strata  (Utica 
shales).  The  channels  reaching  southward  from  the  Gulf  of  St. 
Lawrence,  which  were  extended  by  the  depression,  also  received 
limestone  accumulations,  of  which  remnants  still  are  found  in 
Vermont,  Maine,  and  the  maritime  provinces  of  Canada.  South- 
ward the  principal  area  of  the  Lower  Helderberg  extends  through 
Pennsylvania  to  Virginia,  and  it  recurs  in  western  Tennessee 
and  Missouri,  but  has  not  been  distinguished  in  Wisconsin  or 
Illinois. 

Silurian  strata  are  not  common  in  the  Rocky  Mountain  region, 
and  the  persistence  there  of  comparatively  deep  water  from  Or- 
dovician times  makes  it  very  difficult  to  distinguish  the  series  and 
stages  which  are  so  well  marked  in  the  East,  and  which  were  due 
to  the  oscillations  of  level  and  the  changes  in  the  character  of 
sedimentation  which  occurred  in  the  latter  region. 

Foreign,  —  The  division  into  northern  and  southern  areas  which 
we  found  in  the  Ordovician,  was  maintained  in  Silurian  times,  and 
the  southern  sea  was  as  peculiar  in  its  animal  life  as  it  had  been 
before,  the  northern  being  the  typical  Silurian  which  is  found  in 
the  other  continents.  In  the  west  of  Ireland,  Wales,  northern 
England,  and  Scotland,  Silurian  beds  accompany  and  overlie  the 
Ordovician,  but  the  much  greater  development  of  limestone  points 
to  a  deepening  of  the  water  in  those  seas.  The  Wenlock  lime- 
stone of  Great  Britain,  which  corresponds  to  the  American  Ni- 
agara, is,  like  the  latter,  largely  coralline.  In  Scandinavia  also 
there  is  a  great  development  of  Silurian  limestones,  which  extend 
for  into  Russia.  In  the  latter  country  the  sea  had  retreated  much 
from  its  extension  in  the  Ordovician,  except  toward  the  southeast, 
where  it  was  carried  into  Bessarabia.  Most  of  the  Russian  Silurian 
was  formed  in  an  interior  sea,  connected  with  that  of  southern 
Europe.  In  the  southern  European  countries,  which  display  the 
Bohemian  type  of  development,  or  fades,  the  Silurian  rocks  have 
nearly  the  same  general  distribution  as  the  Ordovician.  The  two 
systems  are  also  associated  in  the  Arctic  islands,  in  China,  north 


LIFE   SYSTEM  389 

Africa,  South  America,  and  Australia.  In  all  of  these  areas,  as  also 
in  North  America,  the  fossils  resemble  those  of  the  northern  Euro- 
pean region,  rather  than  those  of  the  southern.  In  general  the 
Silurian  rocks  are  less  extensively  exposed  at  the  surface  than  the 
Ordovician. 

Close  of  the  Silurian.  —  In  North  America  the  Silurian  passed  so 
gradually  and  gently  into  the  Devonian,  that  it  is  difficult  to  draw 
the  line  between  the  two  systems.  Some  disturbances,  however, 
took  place  in  Ireland,  Wales,  and  the  north  of  England,  for  in  these 
localities  the  Devonian  lies  unconformably  upon  the  Silurian.  In 
other  parts  of  Europe  the  transition  was  gradual. 

THE  LIFE  OF  THE  SILURIAN 

Silurian  life  is  the  continuation  and  advance  of  the  same  organic 
system  as  flourished  in  the  Ordovician,  certain  groups  diminishing, 
others  expanding ;  and  some  new  groups  now  make  their  first 
appearance. 

Plants.  —  Our  knowledge  concerning  the  land  vegetation  of  the 
Silurian  is  not  much  more  definite  than  concerning  that  of  the 
Ordovician.  Most  of  the  remains  referred  to  land  plants  are  of 
disputable  character  ;  the  best  authenticated  is  a  fern  (Neuropfcris) 
from  the  Silurian  of  France. 

Spongida  are  still  common.  A  wide-spread  form  is  Astylospongia 
(PL  III,  Fig.  i). 

Coelenterata.  —  The  Graptolites  have  greatly  diminished,  espe- 
cially the  branching  forms  and  those  with  two  or  more  rows  of 
cells.  Those  that  persist  are,  for  the  most  part,  straight  and 
simple  (III,  2).  The  Hydroid  Corals,  on  the  other  hand,  such 
as  Heliolites,  become  important  elements  of  marine  life  and  in  the 
formation  of  the  reefs.  The  true  Corals  likewise  increase  largely, 
and  play  a  more  important  role  than  in  the  preceding  period. 
The  increase  is  principally  in  the  enlarged  number  of  species  be- 
longing to  much  the  same  genera.  Favosites  is  a  characteristic 
new  genus  (see  III,  3),  and  Halysifes,  the  chain  coral,  is  much 
commoner  than  before. 


THE   SILURIAN   PERIOD 

Echinodermata.  —  In  this  group  we  observe  a  diminution  of  the 
Cystidea,  but  a  marked  increase  of  the  Crinoids  ;  Eucalyptdcrinus 
(see  Fig.  139/1)  is  a  good  example.  Star-fishes  also  have  grown 
more  abundant.  A  new  class  of  the  Echinoderms  now  makes  its 
first  appearance,  the  Blastoidea.  This  class  is  extinct  at  present  and 


SILURIAN  FOSSILS 

FIG.  139.  —  i.  Eucalyptocrinus  crassus,  1/2.    2.  Dalmanites  limulurus. 
Boltoni,  1/3.     (After  Hall.) 


3.  Lichas 


its  structure  is  not  well  understood ;  the  group  remains  rare  in  the 
Silurian  and  Devonian,  first  becoming  important  in  the  Carbonifer- 
ous. The  Echinoids,  or  sea-urchins,  which  were  commoner  than 
before,  have  no  arms,  but  a  closed  spheroidal  or  discoidal  test,  made 
up  of  calcareous  plates,  which  in  all  the  modern  sea-urchins  are 
arranged  in  just  twenty  vertical  rows,  and  are  closely  fitted  to- 
gether by  their  edges,  like  a  mosaic  pavement.  In  the  Palaeozoic 


BRACHIOPODA  391 

sea-urchins  the  number  of  rows  of  plates  is  either  more  or  less 
than  twenty ;  in  some  of  the  Silurian  genera  the  plates  are  loosely 
fitted,  and  slightly  overlapping,  like  fish-scales. 

Arthropoda.  —  Among  the  Crustacea  the  Trilobites  are  still  numer- 
ous, though  decidedly  less  so  than  they  were  in  the  Ordovician ; 
they  represent,  for  the  most  part,  new  species  of  genera  which  have 
survived  from  the  preceding  period.  The  commonest  genera  are 
Calymene,Illcenus,Dalmanites  (Fig.  139/2),  Z/V&w  (Fig.  139/3)  ; 
while  newly  added  are  the  genera  Phacops,  Proetus,  Encrinurus, 
etc.  Eurypterids  continue  to  increase  in  numbers  and  size,  though 
not  reaching  their  maximum  in  either  respect  until  the  Devonian.  In 
these  extraordinary  Crustacea  the  head  is  very  small  and  is  followed 
by  a  long,  tapering  body,  composed  of  thirteejj^vable  segments  ; 
the  last  segment  is  either  a  pointed  spine,  zJ^jEtirypterus,  or  a 
broad  tail-fin,  as  in  Pterygotus.  Five  pairs  of  appendages  are 
attached  to  the  head,  the  bases  of  four  of  which,  on  each  side 
of  the  mouth,  form  the  jaws,  as  in  the  existing  horse-shoe 
crab.  The  first  pair  of  appendages  are  either  short  and  simple 
(JEurypteru3\  Stylonurus),  or  are  much  elongated,  and  armed 
with  pincers  {Pterygotus}.  The  fifth  pair  are  either  very  long,  or 
enlarged  to  serve  as  swimming  paddles.  The  first  body-segment 
carries  a  pair  of  apron-like  appendages,  with  a  narrow  median 
extension,  but  the  other  segments  have  no  appendages.  The 
horse-shoe  crabs  find  their  most  ancient  representative  in  the 
genus  Hemiaspis  of  the  European  Silurian.  Other  Crustacea  are 
much  as  in  the  preceding  period. 

^  Scorpions  have  been  found  in  the  Silurian  of  Europe  and  America, 
and  some  remains  of  Insects  in  the  former  continent.  These  animals 
prove  the  existence  of  a  contemporaneous  land  vegetation,  and  con- 
firm the  doubtful  evidence  of  the  Ordovician  and  Silurian  plants. 

Bryozoa  are  quite  abundant,  and  contribute  in  an  important  way 
to  the  growth  of  the  coral  reefs. 

Brachiopoda  continue  to  be  present  in  multitudes,  but  with  a 
distinct  change  in  dominant  genera  from  those  which  were  com- 
monest in  the  Ordovician.  Especially  characteristic  is  the  increase 
in  the  families  of  the  Spiriferid&t  Pentameridce,  and  Productidce,  all 


392 


THE   SILURIAN   PERIOD 


PLATE  III.   AMERICAN  SILURIAN  FOSSILS 


VERTEBRATA  393 

of  which  continue  prominent  in  the  Devonian.  The  most  impor: 
tant  genera  are  A  try  pa,  Spirifera  (III,  7),  Pentamerus,  andjRJtyn- 
chotreta  (III,  8). 

Mollusca.  —  The  Bivalves  show  no  very  significant  changes 
from  the  Ordovician,  but  the  Gastropods,  especially  such  forms 
as  Capulus  (PL  III,  Fig.  n)  increase  decidedly;  other  well 
represented  genera  of  these  shells  are  Platyostoma  (III,  9)  and 
Cyclonema  (III,  10).  Pteropods  are  smaller  and  less  numer- 
ous than  before  •  a  very  common  form  is  the  little  nail-shaped 
shell,  Tentaculites,  which  is  doubtfully  referred  to  this  group,  but 
may  belong  to  the  Worms.  Among  the  Cephalopods  Orthoceras 
(III,  12)  continues  abundant,  but  Lituites  has  gfown  less  common 
and  Endoceras  has  disappeared,  while  coiled  shells  like  Trochoceras 
(III,  14)  are  not  infrequent.  The  shells  with  curiously  contracted 
mouth  openings,  like  Phragmoceras  (III,  13)  are  more  commonly 
found  than  in  the  Ordovician. 

Vertebrata.  —  The  remains  of  Ostracoderms  and  Sharks  show 
that  Vertebrates  certainly  existed  in  the  Silurian,  but  the  known 
remains  are  so  fragmentary  that  a  description  of  these  curious 
fishes  and  fish-like  animals  will  be  reserved  for  the  following 
chapter. 

EXPLANATION  OF  PLATE  III,  p.  392.  i.  Astylospongia  praemorsa.  2.  Grapto- 
lithes  clintonensis.  3.  Favosi'es  Forbesi.  4.  Lepadocrinus  Gebhardi.  5.  Lingu- 
lella  cuneata.  6.  Orthis  elegantula.  7.  Spirifera  crispa.  8.  Rhynchotreta  cuneata. 
9.  Platyostoma  niagarense.  10.  Cyclonema  cancellatum.  n.  Capulus  angulatus. 
12.  Orthoceras  annulatum,  1/2.  13.  Phragmoceras  parvum,  1/2.  14.  Trochoceras 
desplainense,  1/2.  (Fig.  i  after  Roemer,  Figs.  2-14  after  Hall.) 


CHAPTER   XXV 


THE  DEVONIAN  PERIOD 

THE  name  Devonian,  taken  from  the  English  county  Devonshire, 
was  proposed  by  Sedgwick  and  Murchison  in  1839  5  ^  nas  found 
universal  acceptance  and  has  passed  into  the  geological  literature 
of  all  languages. 

As  in  the  Ordovlcian  and  Silurian,  and  for  the  same  reason,  the 
divisions  of  the  N^w  York  Devonian  are  taken  as  the  standard  of 
reference  for  North  America. 


2.  Chemung  Stage. 

1.  Portage  Stage. 

2.  Hamilton  Stage. 

1.  Marcellus  Stage. 

2.  Corniferous  Stage. 
i.  Schoharie  Stage. 


! 

Upper         4.  Chemung 
Devonian.              Series. 

Devonian 
System. 

Middle 
Devonian. 

3.  Hamilton 
Series. 
2.  Corniferous 

Lower 

Series. 

Devonian. 

I.  Oriskany 
Series. 

DISTRIBUTION  OF  THE  DEVONIAN 

American.  —  In  North  America  the  passage  from  the  Silurian 
to  the  Devonian  is  very  gradual,  the  former  drawing  to  its  close 
without  disturbance  ;  and  there  is  still  some  difference  of  view  as 
to  just  where  the  line  between  the  two  systems  should  be  drawn. 
Many  European  geologists  are  of  the  opinion  that  the  Lower 
Helderberg  should  be  included  in  the  Devonian,  but  in  this 
country  it  is  generally  referred  to  the  Silurian. 

At  the  opening  of  Devonian  times  the  shore  line  of  North 
America  was  approximately  as  follows.  Beginning  on  the  coast  of 
the  Arctic  Ocean,  not  far  from  the  mouth  of  the  Mackenzie,  it 
follows  a  general  southeasterly  course  to  the  region  of  Lake 

394 


AMERICAN   DEVONIAN  395 

Superior,  where  it  turns  westward  to  an  unknown  distance,  the 
covering  of  newer  rocks  preventing  the  tracing  of  the  line  in  that 
direction.  It  reappears  in  northeastern  Iowa,  whence  it  turns 
eastward  across  Illinois  and  then  sweeps  northward  enclosing  a 
great  bay  which  extended  across  Michigan,  Ontario,  and  central 
New  York  nearly  to  the  line  of  the  Hudson,  where  it  encounters 
the  western  shore  of  the  Appalachian  land.  The  islands  over 
southern  Ohio,  Kentucky,  and  Tennessee  made  by  the  Cincinnati 
uplift  at  the  close  of  the  Ordovician,  remained  much  as  they  had 
been  during  the  Silurian.  The  Missouri  island,  on  the  other 
hand,  may  have  been  joined  to  the  Wisconsin  peninsula  and  to 
the  land  mass  of  the  Southwest.  It  has  been  suggested  that  a 
north  and  south  ridge  of  land  extended  from  Wisconsin  all  the  way 
to  South  America,  dividing  the  American  seas  into  eastern,  interior, 
and  western,  just  as  Europe  had  been  separated  into  an  open 
northern  and  a  more  or  less  closed  southern  sea  during  the 
Ordovician  and  Silurian.  This  suggestion  has  not  yet  been 
definitely  confirmed,  but  it  may  represent  the  truth.  In  the 
western  region  extensive  islands  continued  to  exist,  and  were 
probably  somewhat  larger  than  they  had  been  in  the  Ordovician. 

A  region  separate  in  its  geographical  development  was  the 
northeastern  or  Acadian  province,  which  included  the  enlarged 
Gulf  of  St.  Lawrence  and  the  narrow  channels  which  still  ran 
southward  and  southwestward  across  New  England  and  the  mari- 
time provinces  of  Canada. 

In  the  northeastern  bay  of  the  great  Interior  Sea  the  records  of 
the  Devonian  period  begin  with  the  formation  of  a  series  of  thick 
sandstones,  of  which  the  oldest  is  the  Oriskany,  a  calcareous 
sandstone  that  is  rendered  porous  on  weathering  by  the  removal 
in  solution  of  the  calcareous  material  and  of  the  fossils  with  which 
the  rock  is  crowded.  The  Oriskany  is  found  over  the  eastern  half  of 
New  York  and  southward  along  the  Appalachian  range  to  Virginia. 
It  is  also  reported  from  the  folded  area  of  northern  Alabama, 
where  it  overlaps  the  Cambrian  and  Ordovician,  indicating  a  trans- 
gression of  the  sea  over  the  land  in  that  region,  for  the  Silurian 
is  not  represented  there.  Westward  the  Oriskany  extends  into 


396  THE   DEVONIAN   PERIOD 

i 

Ontario  and  recurs  in  southern  Illinois,  while  in  the  eastern  region 
it  is  found  in  east  Canada  and  as  a  very  thick  mass  in  Maine. 

The  Oriskany  was  followed  by  the  grits,  or  fine  conglomerates, 
of  the  Schoharie  stage,  which  have  much  the  same  distribution  as 
the  former  and  are  thickest  along  the  line  of  the  Appalachian 
uplift.  Next,  a  deepening  of  the  water  brought  about  'the  condi- 
tions favourable  for  the  formation  of  the  great  Corniferous  lime- 
stone, which  has  a  much  wider  distribution  than  the  preceding 
stages  and  indicates  a  transgression  of  the  sea  over  many  areas 
that  had  been  low-lying  lands.  This  limestone  extends  from  the 
Hudson  River  across  New  York  to  Michigan,  and  around  the 
islands  of  the  Cincinnati  uplift  into  Indiana,  Illinois,  Kentucky, 
Missouri,  and  Iowa.  It  is  largely  made  of  corals,  sometimes  as 
recognizable  reefs,  a  famous  example  of  which  is  at  the  Falls  of 
the  Ohio,  above  Louisville.  In  the  eastern  province  the  Cornif- 
erous is  represented  at  Gaspe"  (Quebec)  by  2000  feet  of  sand- 
stones and  limestones,  and  by  a  coral  limestone  in  northern 
Vermont. 

A  change  of  conditions  in  the  northeastern  bay  of  the  Interior 
Sea  checked  the  formation  of  limestone,  and  on  a  slowly  subsiding 
bottom  were  laid  down  great  masses  of  shales  and  shaly  sandstones 
(which  constitute  the  Hamilton  series),  with  a  few  feet  of  lime- 
stone at  the  top  in  many  places.  The  Hamilton  has  nearly  the 
same  distribution  as  the  Corniferous,  but  thins  out  much  to  the 
west  and  southland  in  the  Mississippi  valley  is  represented  by 
limestones.  In  the  eastern  province  the  Hamilton  reappears  at 
Gaspe",  where  it  is  displayed  as  a  very  thick  mass  of  sandstones, 
and  in  Nova  Scotia  and  New  Brunswick  as  sandstones  and  shales. 

The  Upper  Devonian,  or  Chemung  series,  as  formed  in  the  north- 
eastern bay,  is  an  exceedingly  thick  mass  of  shales  and  shoal 
water,  ripple-marked  sandstone,  reaching  in  the  Appalachian  ridges 
of  Pennsylvania  a  thickness  of  8000  feet,  but  thinning  away  to  the 
south  and  west.  Indeed,  over  much  of  the  Mississippi  valley  the 
entire  Devonian  system  is  represented  by  a  few  feet  of  black 
shale,  a  circumstance  which  it  is  not  easy  to  explain.  In  the 
Upper  Devonian  sandstone  of  Pennsylvania  are  great  reservoirs  of 


AMERICAN   DEVONIAN  397 

petroleum  and  natural  gas.  Along  the  eastern  shore  of  the  Che- 
mung  sea  was  accumulated  an  immensely  thick  sandstone,  which 
was  formerly  supposed  to  represent  a  distinct  series  and  called  the 
Catskill;  the  maximum  thickness  of  this  sandstone  (7500  feet) 
occurs  in  eastern  Pennsylvania. 

Eastern  New  York  and  Pennsylvania  were,  then,  for  long  ages  a 
slowly  sinking  marginal  sea-bottom,  on  which,  as  in  a  great  trough, 
were  accumulated  immense  masses  of  shallow-water  deposits,  with 
occasional  limestones  when  an  increased  rate  of  subsidence  deep- 
ened the  water.  During  Cambrian  and  Ordovician  times  similar 
conditions  had  prevailed  southward  along  the  line  of  the  future 
Appalachian  range,  but  in  the  Silurian  and  still  more  in  the  Devo- 
nian, sedimentation  became  chiefly  concentrated  in  the  northern 
half  of  the  trough,  the  subsidence  of  the  southern  portion  having 
become  exceedingly  slow  and  intermittent. 

The  course  of  deposition  of  sediments  which  occurred  during 
Devonian  times  in  the  western  portion  of  the  continent  was  so 
entirely  different  from  the  succession  of  sedimentation  in  the  east- 
ern half,  that  it  is  very  difficult  to  correlate  the  subdivisions  in 
the  two  regions,  whose  seas  may  have  been  separated  by  an  un- 
broken land  area.  Far  to  the  north,  in  the  Mackenzie  River 
region  of  Canada  and  coming  down  into  Manitoba,  the  Devonian 
is  represented  by  about  1000  feet  of  limestones  and  shales,  which 
appear  to  belong  to  the  middle  and  upper  part  of  the  system. 
Another  strip  of  Devonian  rocks  follows  the  main  range  of  the 
Canadian  Rocky  Mountains,  extending  southward  into  Montana. 
The  Front  Range  in  Colorado  was  apparently  a  land  area,  for 
there  the  Carboniferous  strata  rest  upon  the  Cambrian  and  Ordo- 
vician, as  is  also  true  of  central  Texas  and  the  Black  Hills  of  South 
Dakota ;  but  in  southwestern  Colorado  the  Devonian  reappears. 
In  parts  of  the  Grand  Canon  region  thin  patches  of  Devonian 
strata  are  found  lying  upon  the  upper  Cambrian  sandstones.  In 
the  Wasatch  Mountains  of  eastern  Utah  2400  feet  of  quartzites  and 
'  limestones  belong  to  the  Devonian.  In  Nevada,  on  the  other 
hand,  was  a  comparatively  deep  and  tranquil  marine  basin,  in 
which  deposition  would  seem  to  have  gone  on  uninterruptedly  from 


398  THE  DEVONIAN   PERIOD 

the  Ordovician  through  the  Carboniferous.  Of  30,000  feet  of 
Palaeozoic  rocks,  6000  feet  of  limestone  and  2000  feet  of  shale 
are  assigned  to  the  Devonian.  Beyond  the  long,  narrow  strip  of 
land  which  lay  along  the  western  side  of  the  Great  Basin,  the 
Devonian  reappears  in  California. 

Foreign.  —  The  European  Devonian  appears  under  two  very 
different  fades,  or  aspects  of  development ;  one  of  these  is  the 
"  Old  Red  Sandstone,"  which  was  laid  down  in  cbsed  basins 
having  restricted  or  occasional  connection  with  the  sea,  and  the 
other  is  the  ordinary  marine  type.  The  period  began  in  Europe 
with  an  advance  of  the  sea  over  the  land  in  many  places,  reaching 
its  maximum  extension  in  the  latter  part  of  the  period,  but  begin- 
ning to  retire  before  the  opening  of  the  Carboniferous.  The 
movement  of  depression  was  at  first  only  sufficient  to  permit  the 
accumulations  of  shallow-water  deposits,  which  in  the  Rhine  dis- 
trict attain  the  great  thickness  of  10,000  feet.  The  Middle  Devo- 
nian in  the  same  region  is  prevailingly  a  great  limestone,  and  the 
Upper  is  made  up  of  limestones  and  slates.  This  subsidence  re- 
moved the  barrier  which  in  Ordovician  and  Silurian  times  had 
separated  the  northern  and  southern  seas,  but  was  accompanied 
by  the  formation  of  closed  basins  farther  to  the  north.  Europe 
then  was  largely  an  open  sea  with  many  islands,  and  where  the 
waters  were  sufficiently  clear  and  free  from  terrigenous  sediment, 
coral  reefs  were  extensively  formed. 

The  marine  Devonian  occurs  in  the  southwest  of  England, 
over  large  areas  of  Germany,  in  northwestern  and  southern 
France,  and  on  an  enormous  scale  in  Russia.  During  the  Silu- 
rian the  sea  had  withdrawn  almost  entirely  from  Russia  west  of 
the  Ural  Mountains.  In  the  Lower  Devonian  the  sea  broke  in 
from  the  north  over  Siberia,  reaching  far  into  central  Asia.  In 
the  Middle  Devonian  a  great  basin  was  formed  by  the  depression 
of  central  Russia,  the  sea  advancing  from  the  north  and  the  east. 

The  "  Old  Red  Sandstone  "  is  of  particular  interest,  because, 
owing  to  the  peculiar  circumstances  of  its  formation,  it  has  pre- 
served a  record  of  Devonian  land  life,  which,  though  fragmentary, 
is  far  more  complete  than  anything  we  possess  from  the  more 


DEVONIAN   LIFE  399 

ancient  periods.  These  strata  were  laid  down  in  closed  basins 
(sometimes,  perhaps,  in  fresh-water  lakes),  which  had  only  a 
restricted  communication  with  the  sea.  The  Old  Red  is  found 
in  south  Wales  and  the  adjoining  part  of  England,  and,  on  a 
much  larger  scale,  in  Scotland ;  also  in  the  Baltic  provinces  of 
Russia,  where  the  fossils  are  mingled  with  those  of  the  marine 
Devonian ;  in  Spitzbergen  and  Greenland  the  same  formation 
recurs.  These  sandstones  are  said  to  be  10,000  feet  thick,  but 
according  to  some  authorities,  the  lowermost  part  of  them  is 
Silurian.  The  so-called  Catskill  of  New  York  is  very  like  the 
Old  Red,  and  contains  similar  fossils. 

The  European  Devonian  is  full  of  the  evidences  of  volcanic 
activity,  in  the  shape  of  great  lava  flows  and  tuffs.  In  central  Scot- 
land the  volcanic  accumulations  exceed  6000  feet  in  thickness. 

Besides  the  Devonian  areas  already  mentioned  in  Asia,  rocks 
of  this  system  are  found  in  China,  the  Altai,  and  in  Asia  Minor. 
They  recur  in  northern  and  southern  Africa,  being  the  most 
ancient  Palaeozoic  rocks  yet  reported  from  the  latter  region.  In 
South  America  occurred  a  great  transgression  of  the  sea,  and 
Devonian  strata  form  larger  areas  of  the  surface  than  those  of 
any  other  Palaeozoic  system.  Shallow-water  deposits  are  found 
in  Bolivia,  over  large  parts  of  Brazil,  especially  the  basin  of  the 
Amazon,  and  in  the  Falkland  Islands.  The  Bolivian  Devonian, 
which  belongs  to  the  lower  and  middle  parts  of  the  system,  con- 
tains a  very  similar  fauna  to  that  of  North  America  and  connects 
the  latter  with  Brazil,  the  Falkland  Islands,  and  south  Africa. 

DEVONIAN  LIFE 

The  life  of  the  Devonian  is,  in  its  larger  outlines,  very  like  that 
of  the  Silurian,  but  with  many  significant  differences,  which  are 
due,  on  the  one  hand,  to  the  dying  out  of  several  of  the  older 
groups  of  animals,  and,  on  the  other,  to  the  great  expansion  of 
forms  which  in  the  Silurian  had  played  but  a  subordinate  role. 

Plants.  — The  fossils  show  that  in  Devonian  times  the  land  was 
already  clothed  with  a  varied,  rich,  and  luxuriant  vegetation  of  the 


400  THE  DEVONIAN   PERIOD 

same  general  type  as  that  whose  scanty  traces  are  found  in  Silu- 
rian strata.  All  the  higher  Cryptogams  are  represented,  and  by 
large,  tree-like  forms,  as  well  as  by  small  herbaceous  plants.  The 
bulk  of  the  flora  is  composed  of  Ferns,  Lycopods  (especially  the 
great  tree -like  Lepid&dendrids)  t  and  Equisetaceaz.  Rhizocarps, 
which  are  now  almost  extinct,  were  then  abundant.  Besides 
these  Cryptogams,  we  find  representatives  of  the  lower  kinds  of 
flowering  plants  in  the  Gyrnnosperms,  including  the  Cycads  and 
perhaps  the  Conifers,  which  presumably  grew  upon  the  higher 
lands.  We  shall  meet  this  same  flora  in  richer  and  more  varied 
display  in  the  Carboniferous  period. 

Foraminifera  and  Sponges  are  not  conspicuous  elements  of  the 
Devonian  fauna. 

Coelenterata.  —  The  Graptolites,  which  were  so  abundant  in 
the  Ordovician  and  had  become  much  less  common  in  the 
Silurian,  are  now  almost  extinct,  only  a  few  simple  species 
occurring  in  the  Lower  Devonian.  The  Corals,  on  the  con- 
trary, expand  and  multiply  enormously  both  in  numbers  and  in 
size.  Most  of  the  Silurian  genera  persist  (though  the  chain- 
coral  Halysites  has  become  extinct),  and  many  new  forms  are 
added.  Heliophyllum  (PI.  IV,  Fig.  i)  is  an  example  of  the  soli- 
tary corals,  and  Phillips  as  tract  and  Acervularia  (IV,  2)  of  the 
reef-builders. 

Echinodermata. —  The  Cystids  have  become  much  rarer  than 
before,  and  are  on  the  point  of  extinction ;  the  Blastoids  are  still 
in  the  background,  and  the  Echinoids  have  not  yet  become  com- 
mon ;  but  the  Crinoids  and  Star-fishes  have  greatly  increased  in 
number  and  variety.  Important  genera  of  the  former  group  are 
Cupressocrinus,  Platycrinus,  Actinocrinus,  etc.  The  multitude  of 
the  crinoids  contributed  largely  to  the  building  up  of  the  calca- 
reous sea-bottom  on  which  they  flourished. 

Arthropoda.  —  The  Trilobites  had  already  begun  to  decline  in 
the  Silurian,  while  in  the  Devonian  the  decline  had  become  very 
much  more  marked,  though  they  were  still  far  from  rare.  New 
species  of  Silurian  genera,  like  Phacops  (IV,  12),  Homalonotus 
(IV,  1 1),  LichaSj  Acidaspis,  etc.,  are  the  commonest.  A  character- 


FOSSILS 


401 


PLATE  IV.   AMERICAN  DEVONIAN  FOSSILS 

I.  Heliophyllum  Halli,  1/2.  2.  Acervularia  Davidson!,  1/2.  3.  Spirifera  pen- 
nata,  3/4.  4.  Athyris  spiriferoides,  3/4.  5.  Rhynchonella  contracta,  5/4.  6.  Pteri- 
nea  flabella,  1/2.  7.  Conocardium  trigonale,  3/4.  8.  Euomphalus  Decervi,  1/3. 
9.  Gomphoceras  mitra,  1/6.  10.  Goniatites  Vanuxemi,  1/3.  n.  Homalonotus 
Dekayi,  1/3.  12.  Phacops  rana,  1/2.  (Figs,  i,  2  after  Rominger.  3-7,  9-12  after 
Hall.  8  after  Meek.) 
2  D 


4O2  THE   DEVONIAN   PERIOD 

istic  of  the  Devonian  Trilobites  is  the  extraordinary  development 
of  spines  which  many  display  on  the  head-  and  tail-shields. 

The  other  Crustacea  make  notable  progress  in  this  period. 
The  first  of  the  Isopoda  and  of  the  long-tailed  Decapoda  (lobster- 
like  forms)  make  their  appearance  in  the  Devonian.  The  Euryp- 
terids  now  attain  their  culmination  in  size,  being  actually  gigantic 
for  Crustacea,  and  some  of  them  are  as  much  as  six  feet  long. 
The  genera  (Eurypferus,  Styfonurus,  and  Pterygotus}  are  the  same 
as  in  the  Silurian.  Insects,  though  still  rare  as  fossils,  are  very 
much  commoner  than  in  the  Silurian ;  they  represent  both  Or- 
thopters  and  Neuropters,  which  are  among  the  primitive  groups. 

Brachiopoda.  —  As  in  the  Silurian,  Brachiopods  continue  to  be 
the  most  abundant  fossils,  both  in  species  and  individuals,  in 
the  Devonian,  from  which  more  than  1000  species  have  been 
described.  Many  Silurian  genera  have  died  out,  and  others,  like 
Orthis  and  Strophomena,  have  become  much  less  common ;  and 
of  others  again,  like  Chonetes  and  Productus,  the  species  are  more 
numerous.  The  most  characteristic  shells  are  those  belonging  to 
the  genera  Spirifera,  especially  the  very  broad  "  winged  "  species, 
(IV,  3),  Rhynchonella  (IV,  5),  Athyris  (IV,  4),  and  those  belong- 
ing to  the  still  existing  family  Terebratulidce,  of  which  Rensellaria 
and  Stringocephalus  are  Devonian  genera. 

Mollusca.  —  Bivalves  and  Gastropods  are  much  as  in  the  Silu- 
rian :  examples  of  the  former  are  Pterinea  (IV,  6)  and  Conocar- 
dium  (IV,  7),  while  large  species  of  the  Gastropod  Euomphalus 
(IV,  8)  are  characteristic.  The  Cephalopods  have  been  revo- 
lutionized ;  the  wealth  of  Nautiloid  shells  which  we  found  in  the 
Silurian  has  been  much  diminished,  though  Orthoceras,  Phrag- 
moceras,  Gomphoceras  (IV,  9),  and  Cyrtoceras  still  persist,  but 
with  fewer  species  tha«n  before,  while  many  other  genera  have 
disappeared.  More  significant  is  the  first  appearance  of  the 
Ammonoid  division  of  the  Tetrabranchiate  Cephalopods,  a  group 
of  shells  which  was  destined  to  attain  extraordinary  development 
in  the  Mesozoic  era.  The  Ammonoids  are  distinguished  by  the 
complexity  of  the  "  sutures,"  or  lines  made  by  the  junction  of  the 
septa  with  the  outer  wall  of  the  shell.  In  the  Devonian  Ammo- 


VERTEBRATA  403 

noids,  of  which  Goniatites  (IV,  10 ;  V,  n)  is  the  common  form, 
the  sutures  are  much  less  complex  than  in  the  Mesozoic  shells. 
Another  member  of  the  group  which  is  far  more  abundant  in 
Europe  than  in  America  is  Clymenia,  the  only  Ammonoid  in 
which  the  siphuncle  is  on  the  inner  side  of  the  spiral.  Bactrites 
has  a  straight  shell,  like  that  of  Orthoceras,  but  with  the  com- 
plex sutures  which  show  it  to  be  an  Ammonoid. 

Vertebrata.  —  One  of  the  most  characteristic  features  of  Devo- 
nian life  is  the  great  development  of  the  aquatic  Vertebrates, 
which  is  so  striking  that  the  period  is  often  called  the  "  Age  of 
Fishes."  So  numerous  and  so  finely  preserved  are  these  fossils 
that  a  satisfactory  account  may  be  given  of  the  structure  and  sys- 
tematic position  of  many  of  the  genera.  This  great  assemblage 
of  fishes  and  fish-like  forms,  it  should  be  remembered,  is  not 
something  entirely  new  in  the  earth's  history,  but  the  wonderful 
expansion  of  types  which  during  the  Ordovician  and  Silurian  had 
remained  very  much  in  the  background. 

Of  the  Devonian  Vertebrates  none  are  more  peculiar  and  char- 
acteristic than  the  Ostracoderms,  which,  though  generally  called 
fishes,  really  belong  to  a  type  much  below  the  true  fishes  and 
more  nearly  allied  to  the  Lampreys,  being  devoid  of  jaws  and  of 
paired  fins.  The  head  and  more  or  less  of  the  body  are  sheathed 
in  heavy  plates  of  bone,  and  the  remainder  of  the  body  and  the  tail 
are  covered  with  scales.  No  trace  of  the  internal  skeleton  is  pre- 
served, and  it  evidently  was  not  ossified.  The  genus  Cephalaspis 
of  this  group  is  curiously  like  a  Trilobite  in  appearance,  though, 
of  course,  the  resemblance  is  entirely  superficial.  The  head- 
shield  is  formed  of  a  single  great  plate  of  bone,  shaped  like  a  sad- 
dler's knife,  with  rounded  front  edge  and  with  the  hinder  angles 
drawn  out  into  spines ;  the  eyes  are  on  the  top  of  the  head  and 
very  close  together.  The  body  is  covered  with  large,  angular 
plates  of  bone,  arranged  in  rows ;  a  small  median  dorsal  fin  and 
a  larger  triangular  tail-fin  make  up  the  locomotor  apparatus. 

Pteraspis  has  a  bony  plate  over  the  snout,  a  large  shield  on  the 
back  and  another  on  the  belly,  with  rhomboidal  scales  covering 
the  rest  of  the  body. 


404 


THE   DEVONIAN   PERIOD 


A  most  extraordinary-looking  creature  is  Pterichthys  (Fig.  140), 
in  which  the  head  and  most  of  the  body  are  encased  in  heavy 
plates,  the  remainder  in  overlapping  scale-like  bones  ;  the  eyes  are 
even  closer  together  than  in  Cephalaspis.  Dorsal  and  tail-fins  are 


FIG.  140.  —  Pterichthys  testudinarius.     (From  Dean,  after  Smith  Woodward.) 

present  and  what  appear  to  be  pectoral  fins.  The  pair  of  append- 
ages referred  to  doubtless  acted  as  fins,  but  they  are  not  com- 
parable to  the  paired  fins  of  the  true  fishes,  being  merely  jointed 
extensions  of  the  head-shield.  These  three  genera,  Cephalaspis, 
Pteraspis,  and  Pterichthys,  have  been  selected  as  types  of  the  Os- 
tracoderms,  each  one  of  which  has  several  allies,  differing  from  it 
in  one  or  other  particular. 

Of  the  true  Fishes  there  is  great  variety  in  the  Devonian.     The 


FIG.  141.  — Cladoselache  Fyleri,  1/5.     (Dean.) 

Selachians  are  well  represented,  one  of  which  is  Cladoselache 
(Fig.  141),  a  small  shark,  from  two  to  six  feet  in  length,  and  the 
most  primitive  known  member  of  the  group.  The  Dipnoi,  or 
Lung  Fishes,  were  important  elements  of  the  Devonian  fish  fauna. 


FISHES 


Dipterus  (Fig.  142),  an  example  of 
this  group,  is  very  like  the  modern 
lung  fishes,  which  have  dwindled  to 
three  genera,  one  in  South  America, 
one  in  Africa,  and  one  in  Australia. 
A  remarkable  series  of  fishes,  the 
Arthrodira,  are   regarded,  though 
with  some  doubt,  as  a  division  of 
the  lung  fishes.     One  of  the  best- 
known  members  of  this  group  is 
the    European    genus     Coccosteus 
( Fig.  1 43 ) ,  in  which  the  head,  back, 
and  belly  are  covered  with  bony 
plates,  but  the  rest  of  the  body  is 
naked.     This   bony  armour  gives 
the  fish  something  of  the  appear- 
ance  of  the   Ostracoderms,   with 
which   group   it   is    often,   though 
erroneously,  classified.     The  back- 
bone is  represented  by  an  unseg- 
mented    rod    (the   notochord,  N, 
Fig.  143),  to  which  arches  of  bone 
are   attached    (N,  ff,  Fig.    143). 
Paired  ventral  fins  were   present, 
but  pectorals  have  not  been  found. 
The  jaws  were  provided  with  teeth, 
which   fuse   into   plates.      In  the 
uppermost  Devonian  of  Ohio  are 
found  some  huge  fishes  allied  to 
Coccosteus,  but   much  larger  and 
more  formidable.     The  most  im- 
portant  of   these    are   Dinichthys 
and    Titanichthys,    the    latter    at- 
taining a  length  of  25  feet. 

A  higher  type  of  Devonian  fish 
is    that   of  the   Crossopterygii,  an 


406 


THE  DEVONIAN   PERIOD 


ancient  group  of  which  but  two  representatives  remain  at  present, 
both  of  them  African.  These  fishes,  like  the  Dipnoans,  have 
"lobate  "  paired  fins  (see  Fig.  144),  i.e.  the  part  of  the  fin  belong- 
ing to  the  internal  skeleton  is  large  and  covered  with  scales,  form- 


FlG.  143.  —  Coccosteus  decipiens.     (Dean,  after  Smith  Woodward.) 

ing  a  lobe  to  which  the  fin-rays  are  attached.  Most  of  the 
Devonian  members  of  the  group  have  massive  rhomboidal  scales, 
but  in  others,  like  Holoptychius,  the  scales  are  thinner,  rounded, 
and  overlapping. 

The  most  advanced  fishes  of  the  period  are  the  Ganoid  mem- 
bers  of  the   Actinopterygians,  which   from   the   Devonian   until 


FIG.  144.  —  Holoptychius  Andersoni.     (Dean.) 

nearly  the  end  of  the  Mesozoic  era  continue  to  be  the  dominant 
type  of  fishes.  Nearly  all  of  them  have  thick,  shining  scales  of 
rhomboidal  shape. 

The  Devonian  fish  fauna  (using  that  term  in  a  very  comprehen- 
sive sense)    is   thus  seen   to   be   a  rich  and  varied  one,  includ- 


AMPHIBIA  407 

ing  Ostracoderms,  Sharks,  Lung  Fishes,  Crossopterygians  and 
Actinopterygians,  each  with  many  representatives  and  mostly  of 
very  curious  and  bizarre  forms.  While  thus  varied  and  plentiful, 
this  assemblage  differs  from  the  modern  fish  fauna  in  the  primitive 
character  of  the  groups  which  are  represented,  and  in  the  entire 
absence  of  the  Bony  Fishes  (Teleosts),  which  now  make  up  the 
vast  majority  of  fishes,  both  fresh-water  and  marine. 

Amphibia.  —  Certain  footprints  which  have  been  recently  re- 
ported from  the  Upper  Devonian  of  Pennsylvania,  show  that  the 
Amphibia,  the  lowest  of  air-breathing  vertebrates,  had  already 
begun  their  career. 


CHAPTER   XXVI 
THE  CARBONIFEROUS  PERIOD 

THE  name  Carboniferous  was  given  in  the  early  part  of  the 
present  century,  when  it  was  supposed  that  every  geological  sys- 
tem was  characterized  by  the  presence  of  some  peculiar  kind  of 
rock.  We  now  know  that  this  conception  is  erroneous  and  that 
workable  coal  seams  have  been  formed  in  all  the  periods  since  the 
Carboniferous.  It  still  remains  true,  however,  that  the  latter  con- 
tains much  the  most  important  share  of  the  world's  supply  of 
mineral  fuel,  upon  which  the  whole  fabric  of  modern  industrial 
civilization  is  founded.  The  great  economic  importance  of  the 
coal  measures  has  caused  them  to  be  most  carefully  surveyed  in 
all  civilized  lands,  a  process  greatly  assisted  by  the  innumerable 
shafts  and  borings  which  penetrate  these  rocks.  One  result  of 
this  gigantic  work  is,  that  the  history  and  life  of  the  Carbonif- 
erous are  better  known  than  those  of  any  other  Palaeozoic  period, 
though  our  knowledge  is  still  very  far  from  complete. 

The  Carboniferous  rocks  are  displayed  in  very  different  aspects 
or  facies  in  the  various  parts  of  the  continent  and  even  in  contigu- 
ous regions.  New  York  no  longer  gives  the  standard  scale,  for 
that  state  has  very  little  that  is  newer  than  the  Devonian.  For 
the  eastern  part  of  the  country  the  sequence  of  strata  in  Pennsyl- 
vania serves  as  the  scale  of  reference,  while  a  very  different  one  is 
needed  for  the  Mississippi  valley.  In  the  Rocky  Mountain  region, 
again,  the  character  of  deposition  deviated  markedly  from  what  oc- 
curred in  the  East,  and  all  over  the  far  West  the  Carboniferous  is 
entirely  marine,  without  coal.  Even  in  this  region,  however,  the  dis- 
tinction between  the  Lower  and  Upper  Carboniferous  may  be  drawn. 
The  following  table,  modified  from  Dana's,  gives  the  succession  in 
Pennsylvania  and  the  middle  West,  Illinois,  Missouri,  Iowa,  etc. 

408 


DISTRIBUTION   OF  THE   CARBONIFEROUS 


409 


Upper 

Carboniferous 
Series. 


Lower 

Carboniferous 
Series. 


PENNSYLVANIA 

4.  Upper  Productive  Stage. 
3.  Lower  Barren  Stage. 
2.  Lower  Productive  Stage. 

1.  Millstone  Grit  Stage. 

2.  Mauch  Chunk  Stage. 


I.  Pocono  Stage. 


MISSISSIPPI  VALLEY 


2.  Coal  Measures. 

1.  Millstone  Grit. 

4.  Chester  Stage. 

3.  St.  Louis  Stage. 

f  Warsaw  Substage. 

2.  Usage     Keokuk  Subst 

Stage.     . 

[  Burlington  Substage. 

.  Kinderhook  Stage. 


DISTRIBUTION  AND  SEQUENCE  OF  CARBONIFEROUS  ROCKS 

American.  —  In  many  parts  of  North  America  the  Devonian  was 
followed  so  quietly  by  the  Carboniferous,  that  it  is  very  difficult 
to  draw  the  line  between  them,  but  in  other  regions  notable  geo- 
graphical changes  occurred.  In  the  Acadian  province,  as  at  Gaspe", 
and  in  Nova  Scotia,  ]^ew  Brunswick,  and  Maine,  there  was  a  time  of 
upheaval  and  erosion  toward  the  end  of  the  Devonian,  followed  by 
a  renewed  depression,  in  consequence  of  which  there  is  an  uncon- 
formity between  the  two  systems.  When  the  Carboniferous  period 
began,  most  of  New  England,  eastern  Canada,  and  Newfoundland 
was  land,  but  the  Gulf  of  St.  Lawrence  was  still  much  larger  than 
at  present,  covering  western  Newfoundland,  most  of  New  Bruns- 
wick, and  part  of  Nova  Scotia,  and  sending  two  long,  narrow  arms 
south  westward  as  far  as  Massachusetts  and  Rhode  Island. 

In  the  Interior  Continental  Sea  wide-spread  changes  occurred, 
but  they  were  accomplished  by  slow  oscillations  of  level,  not 
accompanied  by  violent  disturbances.  In  the  Rocky  Mountain 
region,  there  took  place  a  great  deepening  and  transgression  of 
the  sea,  and  many  of  the  land  areas,  the  outline  of  which  is  still 
very  vaguely  known,  became  extensively  submerged.  In  conse- 
quence of  this  transgression,  the  Carboniferous  strata  rest  uncon- 
formably,  or  in  apparent  conformity,  upon  Cambrian,  Ordovician, 
and  Silurian  rocks.  From  the  Rocky  Mountains  westward  Car- 
boniferous rocks  are  much  more  widely  extended  than  those  of 


410  THE   CARBONIFEROUS   PERIOD 

any  other  Palaeozoic  system,  but  they  cannot  yet  be  compared  in 
detail  with  those  of  the  East.  In  the  central  portion  of  the  In- 
terior Sea  there  was  likewise  a  deepening  and  extension  of  the 
waters,  forming  clear  seas,  in  which  marine  organisms  flourished 
luxuriantly,  and  great  bodies  of  limestone  were  formed  over  areas 
where  the  Devonian  is  very  thin  or  altogether  absent.  The  north- 
ern islands  of  the  Cincinnati  uplift  became  joined  to  the  northern 
land,  forming  a  long,  narrow  peninsula,  while  a  strip  was  added 
along  the  western  side  of  the  Appalachian  land.  The  north- 
eastern part  of  the  Interior  Sea  was  thus  divided  into  two  bays, 
the  larger  one  covering  nearly  all  of  Pennsylvania,  the  eastern 
part  of  Ohio  and  Kentucky,  and  West  Virginia,  the  other  occupy- 
ing the  southern  peninsula  of  Michigan  and  communicating  with 
the  first  by  a  narrow  strait.  So  long  as  marine  conditions  lasted 
in  these  bays,  the  rocks  laid  down  in  them  were  nearly  all  frag- 
mental,  conglomerates,  sandstones,  and  shales  (though  with  occa- 
sional layers  of  limestone),  and  it  is  very  difficult  to  correlate  them 
with  the  thick  masses  of  limestone  accumulated  in  the  clearer  and 
deeper  waters  farther  west. 

The  Carboniferous  strata  are  divisible  into  two  great  series,  the 
Upper  Carboniferous  or  coal  measures  above  and  the  Lower  Car- 
boniferous (often  called  Subcarboniferous)  below,  though  it  must 
not  be  supposed  that  the  formation  of  coal  began  simultaneously 
in  all  the  areas,  for,  as  a  matter  of  fact,  we  know  that  it  did  not. 
In  the  Acadian  province  (Nova  Scotia  and  New  Brunswick)  the 
lower  part  of  the  Carboniferous  system  is  made  up  of  thick  masses 
of  sandstone  and  conglomerate  with  overlying  limestones,  the 
latter  with  inclusions  of  gypsum,  which  indicate  the  occasional 
formation  of  closed  lagoons.  The  thickness  of  these  beds  to- 
gether amounts  to  6000  feet. 

In  Pennsylvania  the  lower  members  of  the  system  are  the 
Pocono  sandstone  and  the  Mauch  Chunk  shales,  which  together 
have  a  maximum  thickness  of  4000  feet.  The  hard  Pocono 
sandstone  forms  the  summit  of  the  plateau  of  that  name  and 
of  many  of  the  Allegheny  ridges.  Westward  and  southward 
the  sandstone  and  shales  thin  away  quickly,  showing  that  their 


AMERICAN  411 

great  thickness  at  the  northeast  was  due  to  the  rapid  deposi- 
tion of  sediment  upon  the  sinking  bottom  of  that  part  of  the  bay. 
In  Ohio  the  Waverly  beds,  representing  the  same  time,  are  a 
mass  of  shales  700  feet  thick,  with  some  sandstone  and  lime- 
stone at  the  top.  In  the  Michigan  bay  were  laid  down  the 
Marshall  beds,  sandstones,  grits,  and  shales  below,  followed  by 
shales  with  some  limestone  and  gypsum,  which  point  to  a  tem- 
porary closing  of  the  straits  and  the  conversion  of  the  bay  into 
a  salt  lake.  The  barrier  was,  however,  removed  and  the  bay 
deepened,  allowing  the  formation  of  a  marine  limestone. 

Westward  from  the  bays  the  deepening  of  the  open  Interior 
Sea  was  followed  by  the  accumulation  of  the  great  masses  of  lime- 
stone which  constitute  the  Mississippian  series,  and  which  were 
formed  from  a  most  luxuriant  growth  of  corals,  brachiopods,  and 
crinoids.  Six  different  stages  and  substages  (see  table)  may  be 
distinguished  in  these  limestones,  and  evidences  of  shifting  coast 
lines  are  not  wanting.  Thus,  the  Kinderhook  extends  farther 
north  and  west  than  the  Osage,  while  the  St.  Louis  represents 
a  renewed  transgression  of  the  sea  northward.  The  Mississip- 
pian limestones  have  a  thickness  of  1200  to  1500  feet  in  southern 
Illinois,  thinning  out  northward,  where  the  coal  measures  overlap 
them  and  rest  upon  the  Devonian.  These  limestones  extend  over 
southern  Indiana,  Illinois,  Iowa,  Missouri,  Kentucky,  Tennessee, 
and  southwestward  to  Arkansas  and  Texas. 

Over  the  eastern  part  of  the  Interior  Sea,  where  there  had  been 
but  scanty  deposition  during  Devonian  times,  a  sinking  sea-bottom 
and  deeper  water  in  the  Lower  Carboniferous  were  favourable  to 
the  formation  of  limestones.  In  West  Virginia  occur  nearly 
1300  feet  of  sandstone  and  shale,  with  800  feet  of  limestone. 
In  Virginia  are  2000  feet  of  limestones,  sandstones,  and  shales, 
with  many  thin  seams  of  coal,  some  of  them  workable.  The 
formation  of  peat  bogs  thus  commenced  in  this  region,  as  also  in 
Nova  Scotia,  before  it  did  in  Pennsylvania,  and  to  distinguish 
them,  these  lower  coal-bearing  beds  are  often  called  the  False 
Coal  Measures. 

At  the  close  of  Lower  Carboniferous  time  occurred  wide-spread 


412  THE   CARBONIFEROUS   PERIOD 

changes  of  level,  which  are  reflected  in  the  character  of  the  rocks, 
and  in  erosion  of  the  lower  strata,  forming  depressions  and  basins 
in  which  peat  bogs  gathered.  In  the  first  instance,  the  change 
consisted  in  a  very  extensive  but  slow  elevation  of  the  sea-bottom, 
raising  it  in  places  into  land.  The  coal  measures  which  some- 
times lie  unconformably  upon  the  Lower  Carboniferous,  were 
inaugurated  by  a  transgression  of  the  sea,  beyond  the  areas  occu- 
pied by  the  Lower  Carboniferous  waters.  The  thick  sheets  of 
conglomerate  and  coarse  sandstones  (Millstone  Grif)  at  the  base 
of  the  coal  measures  were  laid  down  in  the  encroaching  sea. 
Contemporaneous  erosion  (see  p.  272)  and  local  unconformities 
occur  within  the  coal  measures.  The  movement  resulted  in  mak- 
ing a  vast  area  of  low-lying  lands,  which  were  raised  very  little  above 
sea-level  and  upon  which  great  swamps  and  marshes  were  estab- 
lished, where  vegetation  flourished  in  tropical  luxuriance.  A  very 
slow  subsidence,  often  intermittent,  allowed  great  thicknesses  of 
material  to  accumulate,  but  frequently  a  more  rapid  sinking 
brought  in  the  sea,  or  bodies  of  fresh  water  over  the  bogs,  killing 
the  trees  which  grew  there.  We  cannot  yet  determine  how  far 
the  different  coal  regions  represent  separate  basins,  and  how  far 
their  separation  is  due  to  the  subsequent  removal  of  connecting 
strata,  but  even  in  connected  areas  we  find  great  differences  in 
the  nature  and  thickness  of  the  beds.  This  indicates  that  oscil- 
lations of  level  of  different  amounts  took  place  in  particular  parts 
of  the  same  basin.  Thus,  in  one  portion  may  occur  a  coal  seam 
of  great  thickness,  divided  into  two  or  more  layers  by  exceedingly 
thin  "partings"  of  shale.  As  we  trace  the  coal  seam  in  the 
proper  direction,  the  partings  gradually  grow  thicker,  until, 
perhaps,  they  become  strata,  that  intervene  between  very  dis- 
tinct and  quite  widely  separated  coal  seams,  each  of  which  is 
continuous  with  the  corresponding  portion  of  the  thick  seam. 
The  meaning  of  such  a  structure  is,  that  while  one  part  of  the 
bog  subsided  very  slowly,  permitting  the  almost  uninterrupted 
accumulation  of  vegetable  matter,  other  portions  sank  more 
rapidly  and  were  inundated  with  water,  which  deposited  mechan- 
ical sediments  on  the  surface  of  the  submerged  bog. 


ORIGIN   OF  COAL  413 

Hardly  more  than  2%  of  the  thickness  of  the  coal  measures 
consists  of  workable  coal.  The  strata  are  mostly  sandstones, 
shales,  clays,  and  in  some  regions  limestones,  interstratified  with 
numerous  seams  of  coal  of  very  varied  thicknesses.  This  alter- 
nation of  coal  with  mechanical  deposits  does  not  necessarily,  or 
even  probably,  imply  oft-repeated  oscillations  of  level,  but  may  be 
explained  by  assuming  a  general,  slow,  but  intermittent  subsidence. 
After  each  submergence,  we  may  suppose,  the  movement  was 
nearly  or  quite  arrested,  and  the  shallow  water  was  filled  up  with 
sediment,  until  a  bog  could  again  be  formed.  Doubtless,  move- 
ments of  elevation  also  occurred  at  times,  but  the  general  move- 
ment was  downward.  In  the  Nova  Scotia  field  are  76  distinct 
coal  seams,  each  of  which  implies  the  formation  of  a  separate  bog. 
Beneath  most  coal  seams  occurs  what  miners  call  the  "  seat-stone  " 
or  "underclay,"  which  is  ordinarily  a  fire-clay,  or  it  may  be 
siliceous,  but  is  always  evidently  an  ancient  soil.  The  underclay 
is  filled  with  fossil  roots,  from  which  often  rise  the  stumps  of  trees 
that  penetrate  the  coal  seam,  or  may  even  extend  many  feet  above 
it.  The  rock  which  lies  on  a  coal  seam  is  usually  a  shale,  stained 
black  by  organic  matter,  but  may  be  a  sandstone  or  even  a  lime- 
stone, according  to  the  depth  of  water  over  the  submerged  bog. 

That  coal  is  of  vegetable  origin  is  no  longer  questioned.  Such 
a  mode  of  origin  is  directly  proven  by  microscopical  examination, 
which  shows  that  even  the  hardest  anthracite  is  a  mass  of  car- 
bonized but  determinable  vegetable  fibres.  On  the  other  hand, 
there  has  been  much  difference  of  opinion  concerning  the  way  in 
which  such  immense  masses  of  vegetable  matter  were  brought 
together.  Much  the  most  probable  view  is,  that  the  coal  was 
formed  in  position  in  great  peat  bogs,  added  to,  no  doubt,  by 
more  or  less  drifted  material.  The  evidence  for  this  view  is  to 
be  found:  (i)  in  the  great  extent  and  uniform  thickness  and 
purity  of  many  coal  seams,  which  we  cannot  account  for  in  any 
other  way.  Had  the  vegetable  matter  been  largely  drifted 
together,  it  must  have  been  contaminated  with  sediment  and 
could  not  have  been  spread  out  so  evenly  over  great  areas. 
This  objection  to  the  "  driftwood  theory "  becomes  all  the 


414  THE  CARBONIFEROUS    PERIOD 

stronger  when  it  is  remembered  that  the  process  of  converting 
vegetable  matter  into  coal  greatly  reduces  its  bulk,  a  given  thick- 
ness of  coal  representing  only  about  7%  of  the  original  thickness 
of  vegetable  substance.  Thus  a  20- foot  seam  of  coal  implies 
the  accumulation  of  nearly  300  feet  of  plants,  and  it  is  highly 
improbable  that  such  a  mass  could  have  been  evenly  spread  as 
drift  over  hundreds  (or  even  thousands)  of  square  miles,  without 
a  large  admixture  of  mud  or  sand.  (2)  The  very  general  presence 
of  the  underclay  beneath  coal  seams  points  to  the  same  conclusion. 
An  underclay,  as  we  have  seen,  is  an  ancient  soil,  and  is  of  just 
the  same  character  as  that  which  we  find  under  such  modern 
peat  bogs  as  the  Great  Dismal  Swamp  (see  pp.  134-135). 

The  subsidence  of  the  bogs  and  the  deposition  of  sediments 
upon  them  gradually  built  up  the  great  series  of  strata  which  are 
called  the  coal  measures.  The  peat  was  thus  subjected  to  the 
steadily  increasing  pressure  of  the  overlying  masses,  which  greatly 
aided  in  the  transformation  of  the  vegetable  accumulations  into 
coal.  Where  the  coal  measures  have  been  folded,  the  still  greater 
pressure,  aided  by  heat,  and  perhaps  by  steam,  has  resulted  in  the 
formation  of  anthracite.  The  greater  number  of  the  Carboniferous 
bogs  appear  to  have  been  covered  by  fresh  water,  though  some 
were  coast  swamps,  extending  out  into  brackish  or  even  salt  water. 

The  workable  coal  fields  of  North  America,  belonging  to  the 
Carboniferous  system,  are  found  in  several  distinct  areas,  some 
of  which  were  doubtless  separate  basins  of  accumulation,  while 
others  have  become  disconnected  by  denudation. 

(1)  In  the  Acadian  province  the  coal  measures  occur  in  the 
island  of  Cape  Breton,  Nova  Scotia,  and  New  Brunswick  ;  in  Nova 
Scotia  they  are  of  immense  thickness,  7000  feet,  with  6000  feet 
of  underlying  conglomerate.     A  second  basin  of  this  province  is 
near  Worcester  (Mass.),  and  a  third  extends  through  Rhode  Island 
into  southeastern  Massachusetts.     The  latter  basins  are  metamor- 
phic  and  yield  a  very  hard  anthracite. 

(2)  The  great  Appalachian  field  has  an  area  of  more  than 
50,000  square   miles.     It   covers   most   of  central   and  western 
Pennsylvania,  eastern  Ohio,  western  Maryland  and  Virginia,  and 


AMERICAN   COAL   FIELDS  415 

West  Virginia,  eastern  Kentucky  and  Tennessee,  to  northern  Ala- 
bama. In  this  field  the  measures  are  thinner  than  in  Nova 
Scotia  :  the  beds  are  thickest  along  the  Appalachian  shore  line, 
about  4000  feet  in  western  Pennsylvania  and  3000  in  Alabama, 
thinning  much  to  the  westward. 

(3)  In  Michigan  the  measures  are  only  about  300  feet  thick 
and  were  doubtless  laid  down  in  an  isolated  basin. 

(4)  The  Indiana-Illinois  field,  which  extends  into  Kentucky,  is 
from  600  to  1000  feet  thick. 

(5)  The  Iowa-Missouri    field    extends   southward   around    the 
Palaeozoic  island  of  southern  Missouri  into  Arkansas  and  Texas. 
In  Arkansas  the  Carboniferous  system,  as  a  whole,  attains  a  greater 
thickness  than  anywhere  else  in  North  America. 

The  two  latter  fields  are  separated  by  a  very  narrow  interval, 
and  almost  certainly  were  once  continuous ;  the  Indiana-Illinois 
field  was  probably  also  connected  with  the  Appalachian  area 
across  western  Kentucky  and  Tennessee. 

As  the  coal  measures  are  traced  westward  into  Kansas,  Nebraska, 
and  adjoining  states,  we  find  them  dipping  beneath  strata  of  a  very 
much  later  date.  When  they  once  more  return  to  the  surface,  as 
in  the  Rocky  Mountain  region,  they  appear  under  an  entirely  new 
aspect,  being  here  altogether  marine  and  containing  no  coal.  In 
Arizona  and  Utah  u  very  large  area  is  covered  by  Carboniferous 
limestones  and  sandstones,  and  they  form  much  of  the  thickness  of 
the  lofty  plateaus  through  which  the  Colorado  has  cut  its  canons. 
Carboniferous  beds  occur  around  the  Colorado  island,  in  the  Black 
Hills,  westward  in  the  Uinta  and  Wasatch  Mountains,  and  over 
eastern  Nevada.  The  thickness  of  the  beds  increases  from  the 
Rocky  Mountains  westward,  reaching  13,000  feet  of  limestones 
and  quartzites,  for  the  whole  Carboniferous  system,  in  Utah  and 
Nevada,  where  deposition  seems  to  have  gone  on  uninterruptedly 
from  the  Cambrian.  In  western  Nevada  was  the  Pacific  shore, 
beyond  which  Carboniferous  beds  reappear  in  California  and  Brit- 
ish Columbia,  extending  over  the  interior  plateau  of  the  latter 
region  as  far  as  55°  N.  lat.,  and  perhaps  much  farther.  Many 
Arctic  islands  have  Carboniferous  strata. 


41 6  THE   CARBONIFEROUS   PERIOD 

Thus,  in  Carboniferous  times  we  observe  a  striking  difference 
between  the  development  of  the  eastern  and  western  halves  of 
the  continent.  At  first,  there  was  a  deepening  and  extensive 
transgression  of  the  sea.  This  was  followed  in  the  eastern  interior 
region  by  the  upheaval  of  vast  areas  into  low,  swampy  flats.  In  the 
West  the  sea  held  sway  throughout  the  period,  and  the  conditions 
for  the  accumulation  of  coal  were  not  brought  about.  This  was 
done,  as  we  shall  learn,  at  a  far  later  time.  In  the  East  there 
were  numerous  oscillations  of  level,  as  is  shown  by  the  character 
of  the  strata,  though  the  movements  were  very  slow  and  gentle ; 
but  in  parts  of  the  West,  as  around  the  Colorado  island,  there 
were  some  disturbances,  and  unconformities  between  the  Upper 
and  Lower  Carboniferous  beds  have  been  detected.  No  volcanic 
rocks  have  been  found  in  this  system,  East  or  West. 

Foreign.  —  In  Europe  the  Carboniferous  system  is  developed 
in  a  very  interesting  way.  In  the  western  and  central  parts  of 
the  continent  (and  in  Great  Britain)  the  succession  of  strata  is 
very  similar  to  that  of  the  eastern  half  of  North  America,  while 
in  Russia  it  has  more  analogy  with  the  western  half  of  out 
continent  The  changes  of  level  which  opened  the  period  con- 
verted much  of  the  Devonian  sea-bed  into  land,  but  at  the  same 
time  the  sea  broke  in  over  many  of  the  closed  basins  in  which 
the  Old  Red  Sandstone  had  been  laid  down.  From  the  west  of 
Ireland  to  central  Germany,  a  distance  of  750  miles,  stretched  a 
clear  sea,  free  from  terrigenous  sediments,  in  which  flourished  an 
incredible  number  of  corals,  crinoids,  and  other  calcareous  organ- 
isms. From  their  remains  was  constructed  an  immense  mass  of 
limestone,  having  a  thickness  of  6000  feet  in  the  northwest  of 
England  and  of  2500  feet  in  Belgium.  Above  this  great  "  moun- 
tain limestone,"  as  it  is  called  in  England,  come  the  coal  meas- 
ures. In  Scotland  the  limestone  is  replaced  by  shore  and  shallow- 
water  formations,  such  as  sandstones,  with  some  coal.  In  the 
southwest  of  England  and  east  of  the  Rhine  in  Germany,  the 
Lower  Carboniferous  is  represented,  not  by  a  limestone,  but  by  a 
series  of  sandstones  and  slates,  called  the  Culm,  with  the  coal 
measures  above.  In  Russia  the  order  of  succession  is  reversed, 


FOREIGN 


417 


the  productive  coal  beds  being  below  and  the  great  bulk  of  the 
limestone  above.  This  younger  Carboniferous  limestone  is  often 
called  the  Fusulina  limestone,  being  principally  composed  of  shells 
belonging  to  that  genus  of  Foraminifera  (PI.  VI,  Fig.  i).  Great 
areas  of  southern  and  eastern  Asia  are  covered  by  this  limestone, 
which  is  also  largely  developed  in  western  North  America,  extend- 
ing as  far  east  as  Illinois.  In  southern  Europe,  Spain,  the  south 
of  France,  the  Alps,  and  the  Balkan  peninsula,  the  Lower  Carbo- 
niferous is  partly  limestone  and  partly  culm,  while  the  Upper  is 
largely  made  up  of  the  Fusulina  limestone.  In  the  Arctic  Sea, 
Nova  Zembla,  Spitzbergen,  and  Greenland  have  Upper  Carbonif- 
erous limestones. 

The  following  table,  from  Kayser,  displays  the  relations  of  the 
Carboniferous  beds  in  eastern  and  western  Europe. 


LITTORAL  AND 
LACUSTRINE  FACIES 

MARINE  FACIES 

Upper 
Carboniferous. 

Productive 
Coal  Measures 
(Western  Europe). 

Younger  Carboniferous  or 
Fusulina  Limestone 
(Russia,  etc.). 

Lower 
Carboniferous. 

Productive 
Coal  Measures 
(Russia,  etc.). 

Lower  Carboniferous 
Limestone 
(Western  Europe). 

Culm 
(Germany). 

In  western  Europe  the  Carboniferous  period  did  not  run  such 
a  tranquil  course  as  in  North  America,  but  was  broken  by  disturb- 
ances, of  which  the  greatest  were  at  the  close  of  the  Lower  Car- 
boniferous epoch,  when  the  rocks  were  folded  and  upturned  over 
extensive  regions.  These  movements  were  accompanied  and  fol- 
lowed by  volcanic  outbursts,  especially  in  Scotland,  France,  and 
Germany,  and  great  eruptions  occurred  in  China  at  the  end  of 
the  period. 

In  Asia  are  large  areas  of  Lower  Carboniferous  limestone  and 
culm,  and  of  the  Upper  Carboniferous  both  Fusulina  limestone 
and  productive  coal  measures.  China  is  one  of  the  richest  coun- 
tries in  the  world  in  supplies  of  coal. 

2  E 


41 8  THE  CARBONIFEROUS   PERIOD 

Carboniferous  limestones  are  found  in  Morocco,  the  Sahara, 
and  in  southern  Africa. 

The  Carboniferous  limestones  and  coal  measures  are  extensively 
developed  in  the  colonies  of  New  South  Wales  and  Tasmania.  In 
South  America  the  Carboniferous  is  not  nearly  so  extensive  as  the 
Devonian ;  the  Lower  Carboniferous  is  principally  composed  of 
sandstones,  while  the  upper  series,  containing  the  Fusulina  lime- 
stone, has  been  found  in  Peru,  Bolivia,  and  Brazil. 

CARBONIFEROUS  LIFE 

The  life  of  this  period  is  thoroughly  Palaeozoic  and 
along  the  lines  already  marked  out  in  the  Devonian,  fttft  thee 
are  some  notable  changes  and  advances  which  look  toward  the 
Mesozoic  order  of  things. 

Plants. — The  Carboniferous  vegetation  is  of  very  much  the 
same  character  as  that  of  the  Devonian,  but  owing  to  the  peculiar 
physical  geography  of  the  times,  the  plants  were  preserved  as  fos- 
sils in  a  much  more  complete  state  and  in  vastly  larger  numbers. 
The  flora  is  composed  entirely  of  the  higher  Cryptogams  and  the 
Gymnosperms,  no  plant  with  conspicuous  flowers  having  come 
into  existence,  so  far  as  we  yet  know.  By  far  the  most  abundant 
of  Carboniferous  plants  are  the  Ferns,  which  flourished  in  multi- 
tudes of  species  and  individuals,  both  as  tall  trees  and  as  lowly, 
herbaceous  plants.  Many  of  these  ferns  cannot  yet  be  compared 
with  modern  ones,  because  the  organs  necessary  for  trustworthy 
classification  have  not  been  recovered,  and  such  are  named  in 
accordance  with  the  venation  of  the  leaves.  In  other  cases  the 
comparison  with  existing  ferns  may  be  definitely  made,  and  these 
remains  show  that  many  of  the  modern  families  (Marattiacea, 
Ophioglossacea,  etc.)  had  representatives  in  the  Carboniferous 
forests  and  swamps. 

Even  more  conspicuous,  though  much  less  varied,  were  the 
Lycopods,  the  remarkable  character  of  which  has  been  elucidated 
by  the  long-continued  and  laborious  efforts  of  many  investigators. 
While  the  ferns  have  remained  an  important  group  of  plants  to 


LOWER  CARBONIFEROUS  FOSSILS 


419 


10          ^S&TIS 
PLATE  V.   AMERICAN  LOWER  CARBONIFEROUS  FOSSILS 

i.  Lithostrotion  canadense,  3/4.  2.  Pentremites  pyriformis.  3.  Productus  bur- 
lingtonensis,  1/2.  4.  Chonetes  Fischeri.  5.  Spirifera  plena,  1/2.  6.  Onychocrinus 
exsculptus,  i/a.  7.  Melonites  multipora,  1/2.  8.  Archimedes  Wortheni,  1/2. 
9.  Platyceras  infundibulum,  3/4.  10.  Bellerophon  sublaevis.  n.  Goniatites  ixion, 
1/3.  12.  Conularia  missouriensis,  1/2.  13.  Phillipsia  bufo,  3/4.  (Figs.  1-6,  8,  10, 
ii  after  Hall.  Fig.  7  after  Roemer.  Fig.  12  after  White.  Figs,  o  r-i  after  Meek 
and  Worthen.) 


42O  THE  CARBONIFEROUS   PERIOD 

the  present  time,  the  Lycopods  have  dwindled  to  a  few  insignifi- 
cant herbaceous  forms,  but  in  Carboniferous  times  they  were  the 
abundant  and  conspicuous  forest  trees,  at  least  of  the  swampy 
lowlands.  One  of  the  most  characteristic  of  these  trees  was 
Lepidodendron  (PL  VI,  Fig.  12),  of  which  many  species  have  been 
found  in  the  coal  measures.  These  great  club-mosses  had  trunks 
of  2  or  3  feet  in  diameter  and  50  to  75  feet  high,  which  pos- 
sessed the  remarkable  quality,  for  a  'Cryptogam,  of  an  annual 
growth  in  thickness.  At  a  considerable  height  above  the  ground 
the  trunk  divides  into  two  main  branches,  each  of  these  again  into 
two,  and  so  on  (dichotomous  division).  The  younger  parts  of 
the  tree  are  covered  with  long,  narrow,  stiff,  and  pointed  leaves, 
while  the  older  parts  are  without  leaves,  which  have  dropped  off, 
making  conspicuous  scars,  arranged  in  spiral  lines  around  the 
stem.  At  the  ends  of  the  twigs  in  some  species,  or  on  the  sides 
of  the  trunk  and  larger  branches,  in  others,  are  found  the  spore- 
bearing  bodies,  which  have  much  the  appearance  of  pine-cones. 
The  stem  was,  to  a  large  extent,  filled  with  loose  tissue  and  had 
only  a  relatively  small  amount  of  wood. 

Another  very  characteristic  and  abundant  tree  is  Sigillaria ;  it 
is  closely  allied  to  Lepidodendron,  but  has  a  very  different  appear- 
ance. The  trunk  is  quite  short  and  thick,  rarely  branching,  and 
with  a  pointed  or  rounded  tip,  much  as  in  the  great  Cactus ;  the 
leaves  are  similar  to  those  of  Lepidodendron,  but  are  arranged  in 
vertical  rows.  Sigillaria  also  possessed  the  power  of  annual  in- 
crease in  diameter.  Both  Lepidodendron  and  Sigillaria  are  pro- 
vided with  branching  rhizomes,  or  underground  stems,  which  carry 
finger-like  appendages  inserted  into  pits.  Before  the  nature  of 
these  rhizomes  was  understood,  they  were  regarded  as  distinct 
plants  and  named  Stigmaria. 

A  third  group  of  Cryptogams,  the  Equisetacece,  or  Horsetails, 
were  of  great  importance  in  the  Carboniferous  forests.  The 
Catamites  were  decidedly  superior  to  the  existing  horsetails,  not 
only  in  size,  but  in  many  features  of  organization  as  well.  These 
plants  had  tall,  slender  stems  divided  by  transverse  joints,  with 
a  soft  inner  pith,  surrounded  by  a  ring  of  woody  tissue,  which 


SPONGES  42 1 

grew  annually  in  thickness.  The  shape  and  arrangement  of  the 
leaves  differ  much  in  the  various  genera ;  for  example,  they  are 
needle-like  in  Astrophylhtes,  while  in  Annularia  they  are  broad 
and  at  the  base  united  into  a  ring  around  the  stem.  The  shape, 
size,  and  position  of  the  spore-bearing  organs  likewise  differ  in  the 
different  genera,  but  often  resemble  those  of  the  modern  horse- 
tails. The  base  of  the  stem  tapers  abruptly,  and  is  either  con- 
nected with  a  horizontal  rhizome  or  gives  off  a  bundle  of  roots. 
Fragments  of  calamite  stems  are  among  the  commonest  fossils  of 
the  coal  measures  (PL  VI,  Fig.  13). 

The  Flowering  Plants  are  still  represented  only  by  the  Gym- 
nosperms,  of  which  Cordaites  is  a  common  example.  This  plant 
had  a  slender  trunk,  branching  above  into  a  pyramidal  shape  and 
having  long,  broad,  pointed  leaves  like  those  of  certain  lilies  in 
form.  The  trunk  had  a  large,  soft,  inner  pith.  Cordaites  is 
referred  to  the  Cycad  division  of  the  Gymnosperms,  but  has 
certain  resemblances  to  the  Conifers,  which  were  probably  also 
present  in  the  Carboniferous  vegetation. 

The  Carboniferous  flora  is  merely  the  Devonian  flora  some- 
what advanced  and  diversified,  but  the  forests  were  of  the  same 
gloomy,  monotonous  character  as  before.  The  wide  distribution 
and  uniform  character  of  this  flora  are  very  remarkable ;  we  find 
the  same  or  nearly  allied  species  of  plants  spread  over  North 
America,  Europe  (even  in  the  polar  lands,  like  Spitzbergen  and 
Nova  Zembla),  Siberia,  China,  the  Sinai  peninsula,  Brazil,  Aus- 
tralia, and  Tasmania.  This  uniformity  of  vegetation  indicates  a 
corresponding  similarity  of  climate  over  nearly  the  whole  world, 
for  no  trace  of  climatic  zones  can  be  found. 

Foraminifera.  —  For  the  first  time  these  animals  assume  con- 
siderable importance  in  the  earth's  economy.  Many  genera  which 
are  still  living  had  representatives  in  the  Carboniferous  seas,  but 
the  most  conspicuous  and  abundant  is  the  extinct  Fusulina  (PI.  VI, 
Fig.  i),  a  very  large  kind,  with  shells  resembling  grains  of  wheat 
in  size  and  shape.  This  genus  is  especially  developed  in  the 
Upper  Carboniferous. 

Sponges  are  common,  though  rarely  found  in  good  preservation. 


422  THE   CARBONIFEROUS   PERIOD 

Coelenterata.  —  Corals  were  abundant,  and  contributed  largely 
to  the  limestones;  the  genus  Lithostrotion  (PI.  V,  Fig.  i),  which 
is  peculiar  to  this  period,  plays  a  very  prominent  part.  Lophophyl- 
lum  (PI.  VI,  Fig.  14)  is  found  in  the  Upper  Carboniferous. 

Echinodermata  make  up  an  exceedingly  important  part  of  the 
Carboniferous  marine  fauna.  The  Cystids  have  disappeared,  but 
the  Blastoids  have  developed  in  great  numbers  and  are  highly 
characteristic  of  the  Carboniferous  limestones.  As  the  group  is 
entirely  extinct  and  does  not  pass  beyond  the  Carboniferous  sys- 
tem, its  structure  has  much  that  is  problematical  about  it.  The 
delicate,  symmetrical  body  or  calyx  (PI.  V,  Fig.  2),  which  is 
carried  on  a  short  stem,  is  composed  of  a  small,  definite  number 
of  plates,  and  has  five  "  pseudo-ambulacral "  areas,  which  look 
much  like  the  ambulacra  of  a  sea-urchin,  but  really  are  not  at  all 
comparable  to  them.  In  exceptionally  well  preserved  specimens 
numbers  of  delicate  pinnules  are  attached  to  these  areas.  The 
most  abundant  genera  are  Pentremites  (V,  2)  and  Granatocrinus. 

All  other  Echinoderms  of  the  Carboniferous  seas  were  utterly  in- 
significant as  compared  with  the  Crinoids,  which  reach  their  cul- 
mination of  development  in  this  period  :  more  than  600  species 
have  been  described  from  the  Carboniferous  limestones  of  North 
America  alone.  Certain  localities,  such  as  Burlington  (la.)  and 
Crawford sville  (Ind.),  are  famous  for  the  vast  numbers  and  exqui- 
site preservation  of  their  fossil  sea-lilies.  The  crinoid  remains 
occur  in  such  multitudes  that  in  many  places  the  limestones  are 
principally  composed  of  them ;  in  such  places  they  must  have 
covered  the  sea-bottom  like  miniature  forests.  All  the  Carbo- 
niferous Crinoids,  like  those  of  the  earlier  periods,  belong  to  the 


EXPLANATION  OF  PLATE  VI,  p.  423.  i.  Fusulina  ventricosa,  2/1.  (Meek.) 
2.  ^Esiocrinus  magnificus,  1/2.  3.  Spirifera  camerata,  2/3.  (Hall.)  4.  Pro- 
ductuspunctatus,  1/2.  (White.)  5.  Euomphalus  subrugosus.  (Meek.)  6.  Pleuroto- 
mariatabulata,  1/2.  (White.)  7.  Loxonemasemicostata.  (Meek.)  8.  Aviculopecten 
neglectns.  (Meek.)  9.  Allorisma  subcuneatum,  1/2.  (White.)  10.  Sphenophyl- 
lum  Schlotheimi,  1/2.  n.  Pecopteris  orcopteridis,  1/2.  12.  Lepidodendron  cunea- 
tum,  fragment  of  bark,  1/2.  (Rogers.)  13.  Calamites  Suckowi,  1/2.  (Lesquereux.) 
14.  Lophophyllum  proliferum.  (Meek.) 


UPPER  CARBONIFEROUS  FOSSILS 


423 


11 
PLATE  VI.   AMERICAN  UPPER  CARBONIFEROUS  FOSSILS 


424  THE  CARBONIFEROUS   PERIOD 

extinct  division,  Pal(zocrinoideay  none  of  which  passed  over  into 
the  Mesozoic  era.  Of  the  long  list  of  Crinoids  found  in  the  rocks 
of  this  system  may  be  mentioned :  Actinocrinus,  Platycrinus, 
Rhodocrinus,  Onychocrinus  (V,  6),  ALsiocrinus  (VI,  2). 

The  Echinoids,  or  sea-urchins,  are  still  far  less  abundant  than 
the  Crinoids,  but  they  are  much  more  numerous  and  varied,  and 
of  larger  size  than  they  had  been  before.  The  Carboniferous  sea- 
urchins  are,  like  those  of  the  preceding  periods,  members  of  the 
ancient  and  now  extinct  subclass,  Palceoechinoidea,  and  the  com- 
monest genera  are  Melonites  (V,  7),  Oligoporus,  and  Archao- 
cidaris.  In  addition  to  these  should  be  noted  the  beginning  of 
the  modern  subclass,  Euechinoidea,  as  the  still  existing  genus 
Cidaris  is  reported  from  the  Carboniferous. 

The  first  known  Holothuroidea,  or  Sea-cucumbers,  date  from 
this  period. 

Arthropoda. — The  Trilobites  have  become  rare  and  are  soon 
to  die  out  altogether ;  most  of  the  species  belong  to  the  peculiarly 
Carboniferous  genera  Phillipsia  (V,  13)  and  Griffithides,  but  the 
Devonian  ' Proetus  still  persists.  The  Eurypterids  continue,  even 
into  the  coal  measures,  but  they  cannot  compare  in  size  or  num- 
bers with  the  great  Devonian-  forms.  Phyllopods  and  Ostracods 
are  abundant,  and  in  the  coal  measures  are  found  members  of  the 
highest  crustacean  group,  the  Decapods,  of  which  Anthracopaltz- 
mon  is  the  best  known  genus  of  the  time. 

Myriapods  and  Scorpions  are  much  commoner  than  in  the  De- 
vonian, and  the  first  of  the  true  Spiders  are  found  here.  Insects 
likewise  show  a  great  increase  in  numbers,  though  the  Orthopters 
and  Neuropters  are  still  the  principal  orders  represented.  Many 
of  the  Carboniferous  insects  are  remarkable  for  their  great  size, 
some  of  them  measuring  nearly  a  foot  across  the  extended  wings. 
The  character  of  the  vegetation  has  a  very  direct  influence  upon 
insect  life,  and  the  monotonous,  flowerless  Carboniferous  forests 
could  not  have  supported  butterflies,  bees,  wasps,  ants,  or  flies. 
No  insects  of  these  groups  have  been  found  in  the  rocks  of  that 
system,  and  it  is  not  yet  certain  whether  even  beetles  were  then 
in  existence. 


MOLLUSC  A  425 

The  land  life  of  the  Carboniferous  seems  to  be  very  much  more 
varied  and  luxuriant  than  that  of  the  Devonian  and  it  probably 
was  so  in  reality.  It  must  be  remembered,  however,  that  the  im- 
mense development  of  fresh- water  and  marshy  deposits  in  the  Car- 
boniferous was  much  more  favourable  to  the  preservation  of  such 
fossils  than  any  conditions  that  the  Devonian  had  to  offer.  Part, 
at  least,  of  the  striking  difference  in  the  terrestrial  fossils  of  the 
two  periods  is  to  be  accounted  for  in  this  way. 

The  Bryozoa  become  much  more  important  than  they  had 
been  before,  and  contribute  materially  to  the  formation  of  the 
limestones.  Characteristic  Carboniferous  genera  are  the  screw- 
shaped  Archimedes  (V,  8),  and  Chatetes,  while  Fenestella  con- 
tinues to  be  very  abundant. 

The  Brachiopoda  have  undergone  a  marked  diminution,  as 
compared  with  those  of  the  Devonian,  though  they  are  still  very 
common.  Genera  of  long  standing,  like  Atrypa  and  Pentamerus, 
have  died  out,  but  others,  like  Chonetes  (V,  4),  Spin/era  (V,  5  ; 
VI»  3)»  Orthis,  and  Rhynchonetta,  are  still  represented  by  many 
species,  but  most  important  of  all  the  Carboniferous  genera  is 
Productus  (V,  3;  VI,  4),  which  has  a  very  large  number  of 
species,  among  them  P.  giganteus,  the  largest  known  brachiopod. 
The  genus  Terebratula,  which  became  exceedingly  abundant  in 
the  Mesozoic  periods,  has  its  beginning  in  the  Carboniferous, 
though  we  have  already  found  the  family  represented  in  the 
Devonian. 

Mollusca.  —  The  Bivalves  are  somewhat  more  abundant  than 
in  the  earlier  periods.  Examples  of  these  are  Aviculopecten  (VI,  8) 
and  Allorisma  (VI,  9).  Of  Gastropods,  the  same  genera  that 
occur  in  the  Silurian  and  Devonian  are  continued  into  the  Car- 
boniferous, such  as  Bellerophon  (V,  10),  Euomphalus  (VI,  5), 
Pleura fomana  (VI,  6),  Loxonema  (VI,  7),  Platyceras  (V,  9), 
with  the  interesting  addition  of  the  most  ancient  land-shells  yet 
discovered.  The  genus  Conularia  (V,  12),  referred  to  the  Ptero- 
pods,  is  common.  Among  the  Nautiloid  Cephalopods,  Orthoceras 
still  persists,  but  this  group  reaches  its  acme  in  the  number  and 
variety  of  the  coiled  shells,  many  of  which  represent  new  genera, 


426  THE  CARBONIFEROUS   PERIOD 

such  as  Cycloceras,  Trigonoceras,  etc.  The  Nautiloids  have  shells 
ornamented  with  prominent  ridges  or  tubercles.  The  Ammonoids 
continue  to  be  represented  by  Goniatites  (V,  u),  but  the  Carbo- 
niferous species  of  this  genus  display  an  advance  over  those  of  the 
Devonian  in  the  greater  complexity  of  their  sutures,  looking  for- 
ward to  the  remarkable  condition  attained  in  Mesozoic  times. 

Vertebrata.  —  It  is  in  this  group  that  the  most  marked  advances 
of  Carboniferous  life  are  to  be  observed,  and  the  incipient  stages 
of  Mesozoic  development  are  clearly  shown.  The  extraordinary 
and  bizarre  Ostracoderms  have  become  extinct,  though  the  Arthro- 
dirans  continue  into  the  coal  measures. 

The  Selachians  are  numerous  and  varied,  and  some  of  them 
highly  specialized.  Acanthodes  is  a  small  shark  covered  with  a 
dense  armour  of  exceedingly  minute  square  scales,  and  the  fins  are 
supported  by  a  heavy  spine  along  their  anterior  borders.  Another 
remarkable  shark  is  Pleuracanthus  (a  Permian  species  is  shown 
in  Fig.  145),  which  has  many  features  in  common  with  the  Dipnoi, 
such  as  the  shape  of  the  tail,  the  character  of  the  pectoral  fins,  and 
the  bones  which  form  a  roof  for  the  skull,  while  the  skin  is  naked. 
Isolated  fin-spines  and  teeth  show  that  many  other  kinds  of  sharks 
existed  in  the  Carboniferous,  in  some  of  which  the  teeth  were  con- 
verted into  a  crushing  pavement,  adapted  for  a  diet  of  shell-fish. 

The  Dipnoi  continue,  though  in  diminished  numbers,  and  their 
most  prominent  representative  is  the  genus  Ctenodus. 

The  Crossopterygians  are  much  less  abundant  than  in  the  De- 
vonian ;  the  commonest  American  genus  is  Cczlacanthus,  which, 
though  unmistakably  a  member  of  this  group,  has  assumed  the 
form  of  a  bony  fish,  and  looks  much  like  a  chub. 

The  Actinopterygians  are  still  represented  only  by  the  Ganoid 
suborder ;  these  hold  their  own  and  even  increase  their  numbers, 
many  new  genera  replacing  those  of  Devonian  times.  Eurylepis, 
Palceoniscus,  Eurynotus,  and  Cheirodus  are  the  best  known  gen- 
era ;  they  are  all  of  moderate  size  and  in  appearance  are  not  strik- 
ingly different  from  modern  fishes. 

The  Amphibians,  which  we  have  seen  reason  to  believe  existed 
in  the  Devonian,  are  of  greatly  increased  importance  in  the  Car- 


AMPHIBIA  427 

boniferous.  At  the  present  time  the  Amphibia  are  represented  by 
the  dwarfed  and  specialized  frogs  and  toads,  newts  and  salaman- 
ders, which  give  but  an  imperfect  notion  of  the  structure  of  the  ex- 
tinct members  of  the  class.  The  Carboniferous  Amphibia  all  belong 
to  the  extinct  order  Stegocephala,  in  which  the  skull  is  well  covered 
with  a  roof  of  sculptured  bones,  and  which  are  of  moderate  or 
small  size,  not  exceeding  seven  or  eight  feet  in  length  and  mostly 
much  smaller.  The  backbone  is  not  ossified,  the  limbs  are  weak, 
the  tail  short  and  broad,  and  in  many  forms  the  belly  is  pro- 
tected by  an  armour  of  bony  scutes.  An  extraordinary  number  of 
genera  of  Carboniferous  Stegocephala  are  known,  most  of  them 
like  the  Salamanders  in  shape,  but  some  are  elongate,  slender, 
and  of  snake-like  form.  Examples  are  Archegosaurus,  Branchi- 
osaurus,  Dendrerpeton,  Ptyonius,  and  many  others. 


CHAPTER    XXVII 

THE   PERMIAN   PERIOD 

THE  name  Permian  was  given  by  Murchison  in  1841  to  a  series 
of  rocks  which  are  very  extensively  developed  in  the  province  of 
Perm  in  Russia.  In  North  America  the  Permian  followed  upon 
the  Carboniferous  with  hardly  a  break,  so  that  the  distinction 
between  the  two  systems  must  be  made  entirely  upon  the  fossils, 
which  change  very  gradually,  by  drawing  a  somewhat  arbitrary 
line  of  demarcation.  In  consequence,  many  geologists,  especially 
in  this  country,  regard  the  Permian  as  a  mere  subdivision  of  the 
Carboniferous.  Its  relations  with  the  overlying  Triassic  system 
are,  however,  nearly  as  close,  and  by  some  authorities  it  has  been 
referred  to  the  latter.  The  Permian  is,  on  the  whole,  distinctively 
Palaeozoic,  but  it  has  several  features  which  mark  it  out  as  transi- 
tional to  the  Mesozoic. 

TEXAS  PENNSYLVANIA 

p       .       f  Upper  Series.      Double  Mountain  Beds. 

System    I  Middle  Series-   Clear  Fork  Beds- 

[  Lower  Series.     Wichita  Beds.  Upper  Barren  Measures. 

DISTRIBUTION  OF  PERMIAN  ROCKS 

American.  — Toward  the  end  of  the  Carboniferous  there  was  in 
the  low-lying  Appalachian  coal  field  a  slowly  progressive  move- 
ment of  elevation,  resulting  in  the  draining  and  drying  up  of  most 
of  the  region  over  which  the  peat  bogs  had  been  extended.  The 
movement  spread  east,  north,  and  south,  leaving  in  the  middle  of 
the  region  a  smaller  area  in  which  the  conditions  of  the  coal 
measures  continued  very  much  as  before.  In  the  northern  part 
of  the  Acadian  province  Permian  beds  overlie  the  coal  measures 

428 


AMERICAN   PERMIAN 


429 


in  Prince  Edward  Island,  Nova  Scotia,  and  New  Brunswick. 
These  beds  are  soft  red  shales  and  sandstones,  which  were  laid 
down  in  closed  basins,  not  in  the  sea.  In  Pennsylvania  and  West 
Virginia  the  Permian  beds  follow  directly  and  without  any  break 
upon  the  Upper  Productive  stage  of  the  coal  measures :  they  are 
called  the  Upper  Barren  Measures,  and  consist  of  1000  feet  of 
sandstones  and  shales  with  some  limestone  and  a  few  seams  of 
coal.  The  character  of  these  beds  is  entirely  like  that  of  the  coal 
measures,  to  which  they  were  once  referred,  and  their  reference 
to  the  Permian  is  due  to  the  marked  change  which  has  come  over 
the  vegetation.  South  of  West  Virginia  no  Permian  beds  have 
been  found  in  the  Appalachian  area,  owing  to  the  elevation  of  this 
part  of  the  region  at  the  close  of  the  Carboniferous,  but  the 
Permian  occurs  in  Illinois. 

As  we  proceed  westward  and  southward  through  Missouri  into 
Nebraska,  Kansas,  and  Texas,  we  find  the  Permian  assuming  much 
greater  importance,  and  becoming  more  and  more  prominently 
developed  in  extent  and  thickness.  A  study  of  this  region  reveals 
the  fact  that  only  a  part  —  the  lower  —  of  the  Permian  is  developed 
in  the  Acadian  and  Appalachian  areas.  At  the  end  of  the  Lower 
Permian  the  entire  series  of  the  coal  measures  east  of  the  Missis- 
sippi River  was  elevated  and  the  deposition  of  strata  checked. 
In  the  region  beyond  the  Mississippi  the  Permian  beds  thicken 
southward,  attaining  in  southern  Kansas  a  thickness  of  2000  feet, 
and  in  Texas  of  more  than  5000  feet.  The  Ouachita  Mountains 
separate  the  Texas  and  Kansas  areas,  which  were  probably 
covered  by  distinct  bodies  of  water. 

The  lowest  beds  of  the  system,  in  this  western  region,  are  shales 
and  limestones,  which  carry  a  transitional  fauna  of  mingled  Car- 
boniferous and  Permian  types,  followed  by  a  characteristically 
Permian  assemblage.  In  southwestern  Kansas  the  sea  was  then 
excluded  and  salt  lakes  and  lagoons  took  its  place,  in  which  sand- 
stones and  shales,  with  some  dolomite,  were  accumulated,  while 
masses  of  gypsum  and  rock  salt  were  precipitated  from  the  dense 
brines.  These  chemical  deposits  are  now  of  much  commercial 
importance. 


430  THE   PERMIAN   PERIOD 

In  Texas  the  Lower  Permian  (  Wichita  beds)  is  principally  com- 
posed of  sandstones  and  clays,  in  which  some  marine  and  some 
land  fossils  occur.  The  Middle  Permian  {Clear  Fork  beds)  is 
largely  made  up  of  limestones,  and  represents  a  transgression  of 
the  sea  westward,  for  these  strata  often  rest  directly  upon  the  coal 
measures,  without  the  intervention  of  the  Wichita.  In  the  Upper 
Permian  {Double  Mountain  beds)  we  find  evidences  of  a  closed 
basin,  like  that  of  Kansas  and  Oklahoma,  in  the  numerous  beds  of 
gypsum,  and  in  the  quantity  of  salt  which  impregnates  the  shales 
and  clays.  Occasional  incursions  of  the  sea  into  this  basin  are 
indicated  by  the  interbedded  bands  of  fossiliferous  limestone. 
Westward  from  Texas  the  inland  Permian  sea  extended  to  the 
Pacific  land  area,  and  in  it  were  laid  down  the  rocks  of  this  age 
found  in  southern  Utah ;  these  are  masses  of  sandy  shales,  with 
much  disseminated  gypsum  and  very  few  fossils.  In  the  Grand 
Canon  district  the  Permian  beds  are  remarkable  for  their  rich 
colouring.  The  inland  sea  also  extended  northward  around  the 
Colorado  islands,  and  traces  of  its  presence  are  found  to  the  west 
in  the  650  feet  of  Permian  sandstones  and  shales  in  the  Wasatch 
Mountains.  Whether  the  Permian  strata  have  all  been  swept  away 
from  Utah  west  of  the  Wasatch  and  from  Nevada,  or  whether  they 
were  never  deposited  there,  is  a  question  that  cannot  yet  be 
answered.  In  all  this  great  interior  sea  of  the  West  there  is  evi- 
dence that  the  deeper  waters  of  Carboniferous  times  gradually 
gave  place  to  shallow-water  conditions  in  the  Permian. 

Foreign.  —  In  Europe  the  Permian  is  developed  in  two  entirely 
distinct  facies  or  aspects.  In  central  and  western  Europe  the  dis- 
turbances which  closed  the  Carboniferous  resulted,  in  many  places, 
in  a  marked  unconformity  between  the  Carboniferous  and  the 
Permian,  while  in  other  regions  there  is  a  gradual  transition  from 
one  system  to  the  other,  as  in  North  America.  The  result  of 
these  oscillations  was  the  formation  of  a  great  inland  sea,  extend- 
ing from  Ireland  to  central  Germany,  in  which  were  laid  down 
thick  masses  of  red  sandstones,  shales,  and  marls.  Occasionally 
the  ocean  broke  into  this  closed  basin,  bringing  a  marine  fauna 
with  it,  but  only  for  comparatively  short  times.  The  disturbances 


LIFE 


431 


continued  during  the  Permian  period  itself,  in  consequence  of 
which  the  Upper  Permian  has  an  entirely  different  distribution 
from  the  Lower,  overlapping  the  latter  in  various  directions,  and 
resting  upon  Carboniferous  and  older  strata.  Volcanic  eruptions 
were  frequent  and  extensive,  and  great  masses  of  eruptive  rocks 
are  interbedded  in  the  Permian  of  England,  Germany,  and 
southern  France.  In  north  Germany  were  formed  enormous 
deposits  of  rock  salt,  some  of  them  many  thousands  of  feet  in 
thickness. 

Southern  Europe  and  Russia  have  an  entirely  different  aspect  or 
facies  of  the  Permian.  In  the  former  region  the  development  is 
marine,  with  an  abundant  marine  fauna,  which  displays  a  gradual 
transition  from  that  of  the  Upper  Carboniferous.  In  the  Alps  are 
sandstones  and  limestones,  and  in  Sicily  most  interesting  transi- 
tional limestones.  The  Mediterranean  belt  was  thus  part  of  the 
great  ocean,  while  northwestern  Europe  was  covered  by  closed 
seas  and  salt  lakes. 

In  Russia  the  Permian  covers  thousands  of  square  miles ;  its 
lowest  member  shows  the  presence  of  a  sea  which  extended  from 
the  Arctic  Ocean,  along  the  west  flank  of  the  Ural  Mountains,  to 
the  extended  Mediterranean.  Later,  however,  a  closed  basin  was 
formed  in  Russia  also,  in  which  sandstones,  marls,  limestones,  and 
gypsum  were  deposited.  Occasional  irruptions  of  the  ocean  are 
indicated  by  strata  bearing  marine  shells. 

The  marine  facies  of  the  Permian  recurs  in  Asia,  in  the  valley  of 
the  Araxes,  in  Bokhara,  and  in  the  Salt  Range  of  northern  India. 
The  Arctic  islands  of  Spitzbergen  and  New  Scotland  display  Per- 
mian beds. 

The  Permian  of  the  Southern  Hemisphere  is  of  remarkable 
interest,  and  will  be  briefly  considered  in  a  section  at  the  close  of 
the  chapter. 


PERMIAN  LIFE 


We  have  to  note,  in  the  first  place,  that  the  life  of  the  Permian 
is  transitional  between  that  of  the  Palaeozoic  and  of  the  Mesozoic 
eras,  transitional  both  in  the  animals  and  plants.  Here  we  find 


432  THE   PERMIAN   PERIOD 

the  last  of  many  types  which  had  persisted  ever  since  Cambrian 
times,  associated  with  forms  which  represent  the  incipient  stages 
of  characteristic  Mesozoic  types,  together  with  others  peculiar  to 
the  Permian. 

Plants.  —  The  flora  of  the  Lower  Permian  is  decidedly  Palaeozoic 
in  character,  and  that  of  the  Upper  Permian  as  decidedly  Mesozoic, 
so  that  if  the  line  dividing  these  two  great  eras  were  drawn  in 
accordance  with  the  vegetation,  it  would  pass  through  the  Per- 
mian. Even  in  the  Lower  Permian,  however,  the  change  from  the 
Carboniferous  flora  is  a  marked  one.  The  great  tree-like  Lyco- 
pods,  Lepidodendron  and  Sigillaria,  which  were  so  abundant  in  the 
Carboniferous  forests,  have  become  very  rare;  none  of  the  former 
genus  and  only  two  of  the  latter  have  been  found  in  the  Upper 
Barren  Measures  of  Pennsylvania  and  West  Virginia.  The  Cala- 
mites  continue  in  hardly  diminished  numbers  and  importance. 
The  Ferns  are  exceedingly  abundant  and  varied,  and  tree-ferns 
seem  to  be  more  common  than  they  had  been  before.  Especially 
characteristic  genera  of  these  plants  are  Pecopteris,  Callipteris 
(PI.  VII,  Fig.  5),  Cynoglossa,  Neuropteris,  Sphenopteris  (PL  VII, 
Fig.  6),  etc.  The  Gymnosperms  mark  a  notable  advance  ;  in  ad- 
dition to  the  ancient  Cordaites,  are  true  Cycads  (Baiera)  and 
Conifers  ;  of  the  latter  are  found  yew-like  forms,  Walchia,  Saportia, 
with  leaves  nearly  four  inches  wide,  and  Gingko. 

In  the  Upper  Permian  Lepidodendron,  Sigillaria,  and  Calamites 
are  quite  unknown,  though  probably  a  few  stragglers  still  existed, 
and  the  flora  is  made  up  of  Ferns,  Cycads,  and  Conifers,  the 
Angiosperms  still  being  entirely  absent. 

Coelenterata. — The  Corals  are  still  mostly  of  Palaeozoic  type 
and  belong  to  Carboniferous  genera,  but  some  of  the  modern 
Hexacoralla  have  appeared. 

Echinodermata.  —  This  group  has  dwindled  in  the  most  remark- 
able way,  and  instead  of  the  great  forests  of  Crinoids  which 
flourished  in  the  Carboniferous  seas,  are  found  only  occasional 
specimens. 

Arthropoda.  —  The  last  few  stragglers  of  the  genus  Phillipsia 
indicate  the  extinction  of  the  great  Palaeozoic  group  of  Crustacea, 


PERMIAN   FOSSILS 


433 


the  Trilobites,  which  henceforth  we  shall  meet  with  no  more,  and 
the  Eurypterids  have  entirely  disappeared.  Little  is  known  con- 
cerning the  other  Arthropods  of  this  period. 

Bryozoa   are    prominent  in  all   marine    formations,  sometimes 
forming  reefs. 


PLATE  VII.   AMERICAN  PERMIAN  FOSSILS 

I.  Aviculopecten  occidentalis.    2.  Myalinapermiana,  2/3.    3.  Nautilus  Winslowi. 
4.  Medlicottia  Copei,  2/3.    5.  Callipteris  conferta,  1/2.    6.  Sphenopteris  coriacea, 
3/4.    (Figs.  1-4  after  C.  A.  White.     Figs.  5,  6  after  Fontaine  and  I.  C.  White.) 
2  F 


434 


THE  PERMIAN   PERIOD 

The  Brachiopoda  are  still  very  abun- 
dant, especially  in  the  Lower  Permian ; 
they  are  closely  allied  to  those  of  the 
Carboniferous,  and,  as  in  that  period, 
the  Productids  play  the  most  impor- 
tant role,  though  many  of  the  species 
are  peculiar  to  the  Permian. 

Mollusca.  —  In  this  group  very  strik- 
ing changes  are  to  be  noted.     The 
Bivalves  increase  materially  in  variety, 
and  in  addition  to  ancient  genera  like 
Aviculopecten   (VII,  i)  and  Myalina 
;      (VII,   2)    the   Permian   of  India  has 
many  new  forms,  such  as  Area,  Ludna, 
Lima,  etc.     The  Gastropods  require 
no  particular  mention,  except  for  the 
?     great  abundance  of  the  genus  Bellero- 
\    phon  (PL  V,  Fig.   10).     It  is  among 
the  Cephalopods  that  the  great  ad- 
i     vance  takes  place.     Orthoceras  and 
Gyroeeras   continue  from   the    older 
j      periods,    and    many    species    of   the 
|      genus  Nautilus  (VII,  3)   are  added, 
;     but  the  chief  fact  consists  in  the  pres- 
ence of  Ammonoids  with  highly  corn- 
s'    plex   sutures,    far  exceeding,    in  this 
respect,  the  Goniatites  of  the  Carbo- 
niferous, some  of  which  continue  to 
exist  alongside  of  the  more  advanced 
forms.      The    more    important    new 
genera  of  Ammonoids  are  Medlieot- 
tia  (VII,  4),  Ptychites,  Popanoeeras, 
Waagenoceras,  which  have  been  found 
in   Texas,  Sicily,  Russia,  and   India. 
The    presence   of   these   remarkable 
shells  gives  a  strong  Mesozoic  cast  to 
the  Permian  fauna. 


REPTILES  435 

Vertebrata. — The  Fishes  are  still  of  Carboniferous  types,  and 
many  of  the  same  genera  occur,  while  new  ones  are  brought  in. 
To  the  Sharks  are  added  the  curious  Menaspis,  which  is  armed 
with  numerous  long  and  curved  spines.  Among  the  Dipnoi  the 
genus  Ceratodus,  very  closely  allied  to  the  modern  lung  fish  of 
Australia,  makes  its  first  appearance. 

The  Amphibia  are  represented,  as  in  the  Carboniferous,  by  the 
Stegocephala,  and  several  of  the  older  genera  persist,  but  many 
new  forms  appear  for  the  first  time,  several  of  which  much  surpass 
the  Carboniferous  genera  in  size. 

The  most  important  character  that  distinguishes  the  life  of  the 
Permian  from  that  of  all  preceding  periods  is  the  appearance  in 
large  numbers  of  true  Reptiles.  There  is  no  reason  to  suppose 


FIG.  146.  —  Permian  Stegocephalan,  Eryops  megacephalus ,  1/7.     Skull  seen  from 
the  side.     (Cope.) 

that  such  a  variegated  reptilian  fauna  can  have  come  into  exist- 
ence suddenly,  and  their  ancestors  will  doubtless  be  discovered  in 
the  Carboniferous ;  but  while  no  true  reptiles  are  certainly  known 
from  the  latter,  in  the  Permian  they  are  the  most  conspicuous  ele- 
ments of  vertebrate  life.  These  reptiles  represent  two  orders,  one  of 
which,  the  Proganosauria,  is  a  very  central  group,  from  which  many 
other  reptilian  orders  appear  to  be  descended  ;  Proterosaurus  and 
Palaohatteria  are  the  most  important  Permian  genera  of  this  group. 
The  second  order,  that  of  the  Theromorpha,  is  remarkable  for  its 
many  approximations  to  the  structure  of  the  mammals,  as  well  as 
for  the  curious  and  bizarre  forms  which  many  of  its  members 
assume.  This  is  the  only  order  of  reptiles  which  has  so  far  been 
found  in  the  Permian  of  Texas,  but  of  this  group  no  less  than  15 


436  THE   PERMIAN   PERIOD 

genera  and  34  species  have  been  found  in  these  beds.  The  order 
continues  up  into  the  Trias,  where  it  becomes  exceedingly  diversi- 
fied and  more  anomalous  than  ever,  but  is  not  known  from  any 
later  period. 

Examples  of  the  Texas  Theromorphs  are  Pantylus,  Bolosaurus, 
Diadectes,  Empedocles,  etc. 

THE   PERMIAN  OF  THE  SOUTHERN  HEMISPHERE 

The  Permian  has  in  the  Southern  Hemisphere  a  very  remark- 
able and  very  uniform  development,  and  brings  up  some  prob- 
lems of  the  greatest  interest.  In  peninsular  India,  South  Africa, 
Australia,  and  South  America  the  same  phenomena  are  repeated, 
phenomena  which  have  not  been  satisfactorily  explained. 

The  formation  is  particularly  well  developed  in  Australia, 
occurring  from  Tasmania  to  Queensland,  but  is  best  known  in 
Victoria  and  New  South  Wales,  where  these  beds  cover  many 
hundreds  of  square  miles.  The  strata  called  "  Permo-Carbonifer- 
ous  "  are  more  than  2000  feet  thick,  and  in  them  occur  numerous 
beds  of  boulders  of  all  sizes,  some  of  them  weighing  many  tons. 
Many  of  these  boulders  have  been  transported  long  distances 
from  their  parent  outcrops,  and  are  of  angular  shape  and  often 
plainly  scratched  and  scored.  The  boulders  are  held  together  by 
aqueous  deposits,  sand,  or  mud,  and  fossiliferous  marine  strata 
are  associated  with  them.  In  one  locality  are  two  such  marine 
formations,  each  with  several  boulder  beds,  and  between  them 
are  intercalated  230  feet  of  coal  measures,  carrying  from  twenty 
to  forty  feet  of  coal  seams. '  The  pavement  of  older  rocks,  upon 
which  the  boulder-bearing  series  is  laid  down,  is  of  different  ages 
in  different  places,  but  very  frequently  this  pavement  is  grooved 
and  polished,  with  the  formation  of  roches  moutonnees,  as  if  by 
the  passage  of  a  glacier. 

India  has  a  similar  series  of  deposits,  which,  however,  are  not 
marine,  but  were  laid  down  in  an  inland  sea,  apparently  of  fresh 
water.  South  of  the  Himalayas  is  found,  resting  upon  a  founda- 
tion of  metamorphic  rocks,  a  great  mass  of  sandstones  and  shales, 


GLOSSOPTERIS  FLORA  437 

called  the  Gondwdna  series,  which  probably  represents  the  ma- 
rine succession  from  the  Permian  to  the  Jurassic  inclusive.  In  the 
lower  members  are  found  great  boulder  beds,  with  boulders  up  to 
fifteen  feet  in  diameter,  which  have  been  carried  for  several  miles. 
In  some  of  these  beds  the  boulders  are  scored  with  parallel 
grooves  and  beautifully  polished.  As  in  Australia,  the  pavement 
of  older  rocks  is  cut  into  rdches  moutonnees  and  marked  with 
long  parallel  scorings.  The  coal-bearing  rocks  of  India  overlie 
the  boulder  beds,  and  are  regarded  as  Permian. 

In  South  Africa  the  Karoo  series,  partly  Permian  and  partly 
Triassic,  rises  abruptly  near  the  coast,  though  retaining  the  hori- 
zontal position,  in  the  mountain  ranges  called  Quatlambabergen, 
Drakenbergen,  and  Stormbergen.  Here  likewise  have  been  found 
beds  of  scratched  and  polished  boulders  and  pavements  of 
grooved  and  polished  rocks,  just  as  in  Australia  and  India. 

In  South  America,  rocks  having  the  character  and  fossils  of  the 
lower  Gondvvana  beds  have  been  found  in  the  Argentine  Re- 
public and  in  southern  Brazil,  the  latter  with  boulder  beds. 

Permian  life  in  the  Southern  Hemisphere  is  as  characteristic  as 
the  strata  themselves.  The  flora  is  entirely  different  from  that 


FIG.  147.  —  Glossopteris  indica.     (Medlicott  and  Blanford.) 

of  the  Carboniferous,  which  is  also  found  in  Australia,  southern 
Africa,  and  South  America.  This  Permian  flora  contains  no 
Lepidodendron,  Sigillaria,  or  Calamites,  and  has  a  decidedly 
Mesozoic  aspect,  being  made  up  of  Ferns,  Horsetails,  Cycads, 
and  Conifers.  The  most  characteristic  plant  is  the  fern  Glos- 
sopteris, whence  this  vegetation  is  frequently  called  the  "  Glos- 
sopteris Flora,"  and  another  very  abundant  and  widely  spread 
fern  is  Gangamopteris.  Phyllotheca  and  Vertebraria  are  the 
commonest  Horsetails,  and  of  the  Conifers,  Voltzia  (see  Fig.  148, 


438  THE  PERMIAN   PERIOD 

p.  449)  is  the  most  characteristic.  In  northern  lands  the  plants 
of  this  flora  do  not  become  important,  and  some  do  not  even 
make  their  first  appearance,  till  Triassic  or  even  Jurassic  times. 
The  Glossopteris  Flora  has  an  enormously  wide  distribution  ;  it  is 
found  in  Tonquin,  India,  Afghanistan,  southern  Africa,  Australia, 
and  South  America,  and  has  recently  been  discovered  in  Italy. 

The  marine  fauna  of  the  Southern  Hemisphere  is  not  notably 
different  from  that  of  the  Northern.  The  land  fauna  of  Am- 
phibia and  Theromorphous  Reptiles  is  very  nearly  the  same  in 
India,  Africa,  and  South  America;  and  there  is  a  marked  affinity 
with  the  Permian  Vertebrates  of  Russia  and  Texas.  These  ani- 
mals have  not  yet  been  discovered  in  Australia. 

The  facts  regarding  the  Permian  of  the  Southern  Hemisphere 
are  very  puzzling,  and  have  been  much  debated.  The  boulder 
beds  and  the  striated,  polished  pavements  upon  which  they  rest 
are  just  such  evidence  as  has  been  relied  upon  to  prove  the 
reality  of  the  Glacial  Age,  one  of  the  latest  episodes  in  the  history 
of  the  earth.  Hence  many  geologists  have  concluded  that  there 
was  a  glacial  age  in  the  Permian  of  the  southern  continents.  To 
such  an  inference,  however,  there  is  one  serious  objection : 
namely,  the  flora  and  fauna  which  then  flourished  on  those  lands 
and  in  the  adjoining  seas.  In  the  undoubted  Glacial  Age  of  the 
Pleistocene,  not  only  do  the  scorings  and  polishings,  the  moraines 
and  erratic  blocks,  require  the  presence  of  vast  glaciers  for  their 
explanation,  but  the  fossils  are  also  in  harmony  with  this  conclu- 
sion, and  themselves  offer  excellent  evidence  of  a  cold  climate. 
In  the  southern  Permian,  on  the  contrary,  \ve  find  interstratified 
with  the  boulders  beds  containing  every  evidence  of  a  luxuriant 
land  and  marine  flora  and  fauna,  such  as  could  not  possibly  have 
existed  on  or  around  a  continent  buried  under  great  ice-sheets,  as 
is  Greenland  to-day.  That  the  boulder  beds  and  their  polished 
pavements  are  the  work  of  ice,  there  can  be  little  doubt,  but  it 
seems  much  more  likely  that  the  glaciers  were  ice-streams,  descend- 
ing from  highlands,  than  that  Permian  Australia,  for  instance,  was 
buried  under  eleven  successive  ice-sheets.  Probably,  also,  the 
winters  were  sufficiently  cold  to  allow  the  formation  of  floating  ice. 


• 


THE  APPALACHIAN   REVOLUTION  439 

From  the  distribution  of  the  animals  and  plants  of  the  southern 
Permian,  it  is  altogether  probable  that  there  was  land  communi- 
cation between  India,  southern  Africa,  and  South  America,  and  less 
directly  with  Australia. 

CLOSE  OF  THE  PERMIAN 

One  of  the  greatest  revolutions  in  the  history  of  the  North  Amer- 
ican continent  culminated  at  the  end  of  the  Permian,  though  it 
probably  began  in  the  Carboniferous.  With  the  exception  of  the 
mountain  making  at  the  close  of  the  Ordovician,  the  Palaeozoic  era 
in  North  America  had  been  a  time  of  slow,  even  development,  with 
many  oscillations  of  level,  but  without  violent  disturbances,  and  with 
singularly  few  manifestations  of  volcanic  activity.  A  little  more 
land  was  added  to  the  northern  area  during  each  period,  but,  so 
far  as  we  can  trace  it,  the  geography  of  the  Ordovician  does  not 
seem  to  have  been  very  different  from  that  of  the  Carboniferous. 
Throughout  this  long  era,  doubtless  more  than  equal  to  all  subse- 
quent time,  the  Appalachian  geosyncline  had  been  sinking,  though 
with  many  shifts  and  oscillations,  under  an  ever-increasing  load  of 
sediment,  until  the  great  trough  contained  a  thickness  of  30,000  to 
40,000  feet  of  strata.  Eventually  the  trough  began  to  yield  to 
the  lateral  compression  exerted  by  the  shrinking  crust,  and  its 
contained  strata  were  thrown  into  folds,  or  fractured  by  great 
overthrust  faults.  Thus,  in  place  of  a  sinking  sea-bottom  along 
the  shore  of  the  great  Interior  Sea,  rose  the  Appalachian  Moun- 
tains, which  in  their  youth  may  have  been  a  very  lofty  range, 
rivalling  the  Alps  in  height.  This  range  extends  from  the  Hudson 
River  to  Alabama ;  another  range  from  Newfoundland  to  Rhode 
Island,  and  a  third,  the  Ouachita  Mountains  of  Arkansas,  are 
attributed  to  the  same  set  of  disturbances,  which  thus  made  them- 
selves felt  for  a  distance  of  2000  miles.  In  the  West  the  results 
of  this  revolution  are  much  less  clearly  shown.  Either  at  the  close 
of  the  Carboniferous  or  of  the  Permian,  the  Great  Basin  region,  of 
western  Utah  and  eastern  Nevada,  was  upheaved  into  land,  and  at 
the  present  time  the  surface  rocks  over  most  of  this  region  are 


440  THE  PERMIAN  PERIOD 

Carboniferous.  It  is,  however,  not  at  all  impossible  that  the  thin 
Permian  may  have  been  stripped  away  by  denudation,  as  it  has 
been  over  nearly  all  of  the  northern  plateau  of  Arizona. 

At  the  time  when  the  eastern  part  of  the  Great  Basin  was  thus 
converted  into  land,  the  ancient  land  area  of  its  western  border 
was  depressed  beneath  the  sea.  It  is  probable  that  these  two 
movements  were  connected,  though  they  may  have  been  sepa- 
rated by  a  considerable  interval  of  time.  In  Nevada  west  of 
117°  30'  W.  longitude  no  Palaeozoic  rocks  have  been  found,  and 
the  Trias  rests  directly  upon  the  Archaean. 

However  they  may  be  explained,  the  geographical  revolution 
which  closed  the  Palaeozoic  era  was  accompanied  by  the  most 
profound  and  far-reaching  changes  which  have  ever  occurred  in 
the  recorded  history  of  life,  after  which  we  find  ourselves  in  a  new 
world.  It  is  probable  that  the  change  was  a  relatively  rapid  one, 
but  there  are  sufficient  connections  between  the  faunas  and  floras 
of  the  two  eras  to  show  that  the  later  were  derived  from  the 
earlier,  and  that  the  gaps  are  due  to  the  imperfections  of  the 
record. 


CHAPTER   XXVIII 
MESOZOIC  PERIODS  —  TRIASSIC 

THE  Mesozoic  era,  so  far  as  we  can  judge,  seems  to  have  been 
much  shorter  than  the  Palaeozoic ;  in  North  America  Mesozoic 
rocks  are  very  much  more  important  and  widely  spread  in  the 
western  half  of  the  continent  than  in  the  eastern.  The  latter 
region  was,  in  a  measure,  completed  by  the  Appalachian  revolu- 
tion, and  subsequent  growth  consisted  merely  in  the  successive 
addition  of  narrow  strips  to  the  coast-line,  but  in  the  West  many 
great  changes  were  required  to  bring  the  land  to  its  present  condi- 
tion. 

The  life  of  the  Mesozoic  constitutes  a  very  distinctly  marked 
assemblage  of  types,  differing  both  from  their  predecessors  of  the 
Palaeozoic  and  their  successors  of  the  Cenozoic.  In  the  course  of 
the  era  the  Plants  and  marine  Invertebrates  attained  substantially 
their  modern  condition,  though  the  Vertebrates  remain  through- 
out the  era  very  different  from  later  ones.  Even  in  the  Verte- 
brates, however,  the  beginnings  of  the  newer  order  of  things  may 
be  traced.  In  the  earlier  two  periods,  the  Triassic  and  Jurassic, 
vegetation  is  almost  confined  to  the  groups  of  Ferns,  Cycads,  and 
Conifers,  but  with  the  Cretaceous  come  in  the  Angiosperms,  both 
Monocotyledons  and  Dicotyledons,  and  since  then  the  changes  have 
been  merely  in  matters  of  detail. 

With  few  exceptions,  the  ancient  Tetracoralla  have  all  disap- 
peared, and  the  modern  Hexacoralla  take  their  place.  The 
Echinoderms  are  all  markedly  different  from  the  Palaeozoic  types. 
The  Cystids  and  Blastoids  have  died  out,  and  the  Crinoids  have 
been  revolutionized,  the  Palceocrinoidea  being  replaced  by  the 
Neocrinoidea.  Likewise  the  modern  sea-urchins,  Euechinoidea, 
replace  the  ancient  Palceoechinoidea,  and  many  Mesozoic  genera 

441 


442  THE   MESOZOIC   PERIODS 

of  the  former  group  are  still  living  in  our  modern  seas.  The  Star- 
fishes also  assume  their  modern  condition.  Brachiopods  are  far 
less  abundant  and  diversified  than  they  had  been  in  the  Palaeozoic, 
and  belong,  for  the  most  part,  to  different  families,  while  the  Bi- 
valve and  Gastropod  Mollusca  increase  to  a  wonderful  extent. 
Especially  characteristic  are  the  marvellous  wealth  and  variety  of 
the  Ammonoid  Cephalopods,  which  disappear  at  the  close  of  the 
era.  The  Dibranchiate  Cephalopods,  with  internal  shells,  make 
their  first  appearance  in  the  Mesozoic,  and  one  group  of  them, 
the  Belemnites,  is  almost  exclusively  confined  to  the  era.  The 
Arthropods  show  the  same  revolutionary  changes.  Among  the 
Crustacea,  the  Trilobites  and  Eurypterids  have  gone  out,  but  all 
the  modern  groups  are  well  represented,  though  many  of  the 
Mesozoic  genera  are  no  longer  to  be  found  in  the  seas  of  to-day. 
Insects  reach  nearly  their  modern  condition,  so  far  as  the  large 
groups  are  concerned,  butterflies,  bees,  wasps,  ants,  flies,  beetles, 
etc.,  being  added  to  the  older  orthopters  and  neuropters. 

Fishes  become  modernized  before  the  close  of  the  era,  the 
Bony  Fishes  having  acquired  their  present  predominance.  The 
Amphibia  take  a  subordinate  place,  and  after  flourishing  for  a 
time,  the  great  Stegocephala  die  out,  leaving  only  the  pygmy  sala- 
manders and  frogs  of  the  present.  Birds  and  Mammals  make 
their  first  appearance,  the  former  advancing  rapidly  to  nearly  their 
present  grade  of  organization,  though  not  reaching  their  present 
diversity,  while  the  mammals  remain  throughout  the  era  very 
small,  primitive,  and  inconspicuous.  The  most  significant  and 
characteristic  feature  of  Mesozoic  life  is  the  dominance  of  the 
Reptiles,  which,  in  size,  in  numbers,  and  in  diversified  adaptation 
to  various  conditions  of  life,  attain  an  extraordinary  height  of  de- 
velopment. The  Mesozoic  is  called  the  "  Era  of  Reptiles,"  because 
these  were  the  dominant  forms  of  life.  They  filled  all  the  roles  now 
taken  by  birds  and  mammals ;  they  covered  the  land  with  gigantic 
herbivorous  and  carnivorous  forms,  they  swarmed  in  the  sea,  and, 
as  literal  flying  dragons,  they  dominated  the  air.  At  the  present 
time  there  are  only  four  orders  of  reptiles  in  existence,  and  of 
these  only  the  crocodiles  and  a  few  snakes  attain  really  large  size. 


THE  TRIASSIC   PERIOD  443 

In  the  Mesozoic  era  no  less  than  twelve  reptilian  orders  flourished, 
and  nearly  all  of  them  had  gigantic  members.  Some  were  the 
largest  land  animals  that  ever  existed,  and  the  sea-dragons  rivalled 
the  whales  in  size.  Nothing  so  clearly  shows  that  the  Mesozoic  era 
is  a  great  historical  fact,  as  the  dominance  of  its  reptiles. 

The  Mesozoic  climates  offer  some  difficult  problems.  In  general, 
the  climate  was  mild,  as  is  shown  by  the  plants  found  in  the  Meso- 
zoic rocks  of  Arctic  lands,  for  in  Greenland,  Alaska,  and  Spitzbergen 
was  a  luxuriant  vegetation  of  warm  temperate  type.  On  the  other 
hand,  certain  geologists  have  maintained  the  existence  of  distinct 
climatic  belts  in  the  Mesozoic,  indicating  equatorial,  northern,  and 
southern  zones,  but  by  others  this  interpretation  is  denied. 

The  Mesozoic  era  comprises  three  periods,  —  the  Triassic, 
Jurassic,  and  Cretaceous. 

THE  TRIASSIC  PERIOD 

The  Triassic  period  is  so  named  from  the  very  conspicuous 
threefold  subdivisions  of  this  system  of  strata  in  Germany,  where 
its  rocks  were  first  studied  in  detail,  and  where  they  occupy  a 
greater  area  than  in  any  other  European  country  ./'.The  German 
Trias  is,  however,  not  the  usual  and  normal  faoi^of  the  system, 
but  a  very  peculiar  one,  and  cannot  be  taken  as  the  standard  of 
comparison  for  most  other  countries. 

The  Trias  of  North  America  is  displayed  under  three  very 
different  facies,  —  that  of  the  Pacific  coast,  which  is  marine;  that 
of  the  interior,  which  is  lacustrine  ;  and  that  of  the  Atlantic  border, 
which  is  estuarine.  Owing  to  the  absence  of  fossils  common  to 
all  it  is  not  yet  possible  accurately  to  correlate  the  three  facies, 
but  the  divisions  of  the  Pacific  and  Atlantic  borders  are  given  in 
the  following  table  :  — 


Triassic 
System. 


PACIFIC  BORDER          ATLANTIC  BORDER 

Baiuvaric  Series. )  i    c-    • 

„.    ..    c    .  [  ?  Newark  Series. 

Tirohc  Series.      ) 

Dinaric  Series. 
Scythic  Series. 


444  THE  TRIASSIC   PERIOD 

DISTRIBUTION  OF  THE  TRIAS 

American. —  In  the  early  part  of  the  Triassic  period  both  North 
and  South  America  extended  farther  eastward  than  they  do  now. 
It  is  a  remarkable  fact  that  no  marine  deposits  of  this  period 
are  known  to  exist  on  the  Atlantic  slope  of  either  continent, 
but  numerous  isolated  bodies  of  brackish  and  fresh-water  deposits 
occur  in  lines  approximately  parallel  to  the  present  eastern  coasts 
of  North  and  Central  America.  At  some  time  during  the  period, 
perhaps  in  its  latter  half,  there  were  formed  in  the  northern  con- 
tinent a  series  of  long,  narrow  troughs,  running  closely  parallel  to 
the  trend  of  the  Appalachian  Mountains,  though  separated  from  that 
range  by  the  ridges  of  crystalline,  metamorphic  rocks  which  had 
formed  the  western  border  of  the  Appalachian  land  in  Palaeozoic 
times.  In  these  troughs  water  was  accumulated,  forming  some- 
times tidal  estuaries,  sometimes  fresh-water  lakes,  streams,  and 
bogs.  A  great  mass  of  rocks  was  laid  down  upon  the  slowly 
subsiding  bottoms  of  the  troughs.  The  thickness  of  these  rocks 
has  not  yet  been  definitely  measured,  but  is  very  great,  —  so  great 
that  some  authorities  maintain  that  their  formation  must  have 
begun  in  the  Permian,  at  least  in  Pennsylvania. 

At  the  present  time  the  Triassic  rocks  of  the  Atlantic  border 
are  found  in  separate  patches  from  Nova  Scotia  to  South  Carolina, 
the  largest  continuous  area  extending  from  the  Hudson  River 
across  New  Jersey,  southeastern  Pennsylvania  and  Maryland,  into 
Virginia.  Other  areas  occur  in  Nova  Scotia,  Prince  Edward 
Island,  Massachusetts,  Connecticut,  and  several  scattered  ones 
in  Virginia  and  North  Carolina.  The  rocks  are  prevailingly  red 
sandstones,  but  coarse  conglomerates  and  breccias  are  common 
at  the  base  of  the  series ;  ferruginous  shales  are  frequently  inter- 
bedded  with  the  sandstones,  and  some  thin  beds  of  impure  lime- 
stones are  locally  developed.  In  Virginia  and  North  Carolina  the 
conditions  of  Carboniferous  times  were  once  more  established,  and 
in  the  peat  bogs  and  swamps  of  those  regions  workable  coal  of 
good  quality  was  accumulated.  This  coal  can  be  distinguished 
from  that  of  the  preceding  periods  by  the  character  of  the  plants 


AMERICAN  445 

of  which  it  is  composed.  The  rocks  of  this  Newark  series  were 
evidently  formed  from  the  waste  of  the  granite  and  crystalline 
schists  in  the  neighbouring  hills,  for  the  sandstones  are  largely 
felspathic  and  micaceous.  The  sedimentary  rocks  of  the  Appa- 
lachian range  did  not  contribute  to  the  formation  of  the  Triassic 
deposits,  their  drainage  being  to  the  west,  for  the  metamorphic 
ridges  to  the  eastward  effectually  cut  off  the  Appalachian  streams 
from  the  Triassic  estuaries. 

It  is  still  a  question  whether  these  inland  bodies  of  water  were 
originally  separated  or  continuous,  though  it  seems  probable  that 
all  the  areas  lying  to  the  southwest  of  the  Hudson  were  thus  con- 
nected, while  those  of  the  Connecticut  valley  and  Nova  Scotia 
may  have  been  formed  in  another  continuous  body  of  water.  Evi- 
dences of  tidal  action  are  to  be  seen  in  the  rocks  of  both  of  these 
estuaries.  Fossils  are  rare  in  these  rocks,  and  none  of  marine 
origin  have  been  found  ;  land  plants  and  the  footprints  of  land 
animals  are  the  commonest  fossils,  but  in  some  localities  fishes 
are  quite  numerous. 

The  sedimentary  rocks  of  this  estuarine  Trias  are  much  faulted, 
and  some  of  the  dislocation  appears  to  have  taken  place  while  the 
sediments  were  still  in  process  of  deposition.  The  beds  are  also 
cut  by  many  dikes  of  diabase,  and  sheets  of  the  same  are  inter- 
calated between  the  strata.  Some  of  these  sheets  are  contempo- 
raneous lava  flows,  and  indicate  much  volcanic  activity  during 
Triassic  times ;  others  are  intrusive,  and,  with  the  dikes,  belong  to 
a  later  series  of  disturbances.  These  igneous  rocks  everywhere 
accompany  the  Triassic  strata,  from  Nova  Scotia  to  North  Caro- 
lina. 

The  rarity  of  fossils  makes  the  exact  reference  of  the  Newark 
series  a  matter  of  uncertainty,  but  the  evidence  favours  the  con- 
clusion that  these  rocks  belong  in  the  upper  part  of  the  Triassic 
system. 

In  the  interior  region  a  large  part  of  the  continent  was  covered 
by  a  great,  but  shallow  inland  sea,  which  must  have  been  shut  off 
from  the  ocean.  What  are  now  the  mountain  ranges  of  Colorado 
formed  one  long  island,  reaching  from  Wyoming  to  New  Mexico, 


446  THE  TRIASSIC   PERIOD 

On  each  side  of  that  island  were  the  waters  of  the  shallow  in- 
land sea  that  extended  westward  across  the  site  of  the  Wasatch 
Mountains  to  the  eastern  shore  of  the  Great  Basin  land,  which 
was  upheaved  at  the  close  of  the  Carboniferous  or  Permian ;  east- 
ward the  sea  extended  to  an  unknown  distance.  Western  Texas, 
northwestern  New  Mexico,  and  the  adjoining  part  of  Arizona  were 
covered  by  these  waters,  while  nearly  all  of  Mexico  was  land, 
except  an  apparently  isolated  body  of  water  in  Sonora.  The 
Mexican  land,  joined  to  the  Great  Basin  land,  enclosed  the  sea  on 
the  south  and  west;  northward  its  waters  ended  a  little  beyond 
the  forty-ninth  parallel  of  latitude,  and  did  not  extend  so  far  west 
as  the  Selkirk  and  Gold  ranges  of  British  Columbia.  In  some 
places  this  inland  sea  was  established  by  transgression  over  ancient 
lands;  in  Wyoming,  for  example,  Triassic  beds  rest  upon  pre- 
Cambrian  crystalline  rocks. 

Over  this  great  area  were  deposited  a  series  of  rocks,  chiefly 
sandstones,  containing  much  gypsum  and  some  salt,  an  evidence 
of  salt-lake  conditions ;  but  in  southwestern  Colorado,  northwest- 
ern New  Mexico,  and  western  Texas  fresh-water  conditions  pre- 
vailed, at  least  toward  the  end  of  the  period.  The  thickness  of 
the  strata  varies  from  600  to  2000  feet.  In  this  inland  Trias  very 
few  fossils  of  any  sort  have  been  found,  and  none  of  them  are 
marine.  In  many  places  the  reference  of  these  beds  must  be 
made  upon  purely  stratigraphical  grounds. 

On  the  western  shore  of  the  Great  Basin  land  we  find  an  en- 
tirely different  state  of  things  from  that  which  obtained  on  the 
Atlantic  border  or  in  the  interior  region.  The  part  of  the  Great 
Basin  area  which  had  been  land  during  the  Palaeozoic  went  down, 
allowing  the  ocean  to  extend  across  the  site  of  the  Sierra  and  to 
cover  western  Nevada,  and  in  British  Columbia  it  submerged  the 
land  eastward  across  the  mountains.  The  Pacific  coast-line  was 
thus  considerably  to  the  east  of  its  present  position,  and  from  it 
a  gulf  extended  into  southeastern  Idaho.  Marine  Trias  also  re- 
curs on  the  shores  of  Alaska.  The  Pacific  coast  Trias  has  a  maxi- 
mum thickness  of  4800  feet,  and  contains  representatives  of  nearly 
all  the  series  which  make  up  the  system.  Its  successive  faunas 


FOREIGN  447 

indicate  extensive  changes  in  the  physical  geography  of  the  lands 
around  the  Pacific.  In  the  lower  Trias  (Scythic  series)  the  connec- 
tion of  our  Pacific  shore  was  with  the  Indian  and  Arctic  regions  ;  in 
the  middle  or  Dinaric  series  the  connection  with  the  Arctic  region  is 
still  close,  but  migration  from  the  Mediterranean  region  had  begun, 
while  in  the  Tirolic  series  the  relation  is  most  intimate  with  the 
Indian  and  Mediterranean  regions  of  the  Old  World.  Hardly 
any  of  the  uppermost,  or  Bajuvaric,  series  is  found  on  the  west 
coast  of  North  America. 

Foreign.  —  In  Central  America  (Honduras)  has  been  found 
another  area  of  estuarine  Trias  like  that  of  our  Atlantic  States. 
All  South  America  east  of  the  Andes  was  above  the  sea,  for  ma- 
rine Trias  is  known  only  on  the  west  side  of  the  Cordilleras. 

In  Europe  the  Trias  is  displayed  in  two  very  different  facies, 
that  of  central  Europe,  the  production  of  inland  seas  and  salt 
lakes,  and  that  of  southern  Europe,  or  the  Alpine  facies,  which  is 
marine.  In  the  former  region  the  conditions  of  the  Permian 
were  largely  continued,  though  the  situation  of  the  basins  was 
often  different  from  what  it  had  been  in  the  Permian.  In  Ger- 
many Triassic  rocks  cover  a  very  wide  area,  extending  across  the 
southern  and  central  parts  of  the  empire  from  Poland  into  France. 
These  rocks  are  very  obviously  divided  into  three  series,  —  a  lower 
division  of  sandstones  and  clays,  the  Bunter  Sandstone;  a  middle 
calcareous  division,  the  Muschelkalk;  and  an  upper  sandy  division, 
the  Keuper.  The  upper  and  lower  divisions  are  those  of  the 
closed  basins,  with  some  formation  of  coal  in  the  Keuper,  while 
the  Muschelkalk  represents  an  invasion  of  the  sea  from  the  south, 
and  contains  a  considerable  marine  fauna.  Triassic  deposits  ex- 
tend from  the  north  of  Ireland  across  England,  and  in  southern 
Sweden  is  coal-bearing  Keuper.  In  these  northern  lands  the 
Muschelkalk  is  absent,  and  evidently  the  transgression  of  the  sea 
did  not  extend  to  them.  The  sandy  and  clay  deposits  of  Eng- 
land, France,  and  Germany  were  laid  down  in  very  shallow 
waters,  while  deposits  of  gypsum  and  salt  indicate  the  pres- 
ence of  salt  lakes.  Judging  from  modern  conditions,  we  may 
infer  that  in  Permian  and  Triassic  times  the  climate  of  north- 


448  THE  TRIASSIC   PERIOD 

western  Europe  was  warm  and  dry,  evaporation  exceeding  rain- 
fall. 

In  southern  Europe  the  Trias  was  formed  under  very  different 
conditions.  A  great  sea  covered  all  that  region,  extending  from 
Spain  over  the  site  of  the  Alps  eastward  to  the  Himalayas,  and 
in  this  region  the  deposits  are  mostly  limestones,  with  very  rich 
marine  faunas.  In  the  lower  Trias  the  Mediterranean  was  not 
connected  with  the  Pacific  or  Arctic  oceans,  but  in  the  middle 
division,  as  we  have  seen,  the  connection  was  made,  and  Mediter- 
ranean species  extended  their  range  to  California.  Around  the 
borders  of  the  Pacific- Arctic  seas  were  laid  down  the  Triassic 
deposits  of  northern  and  eastern  Siberia,  Spitzbergen,  Japan,  New 
Zealand,  and  the  west  coasts  of  North  and  South  America. 

The  peninsular  part  of  India  was  still,  to  a  great  extent,  cov- 
ered by  the  inland  sea  which  had  been  formed  there  in  the  Per- 
mian period  (see  p.  437),  and  part  of  the  great  Gondwana  series 
is  referable  to  the  Trias.  In  South  Africa  part  of  the  Karoo 
series  is  Permian,  but  most  of  it  is  Triassic  ;  it  consists  of  8000  to 
10,000  feet  of  shales  and  sandstones  laid  down  in  an  inland  sea. 
That  the  land  connection  with  India  persisted  is  indicated  by  the 
continued  similarity  of  the  land  animals  and  plants. 

THE  LIFE  OF  THE  TRIASSIC 

Triassic  life  is  entirely  different  from  anything  that  had  pre- 
ceded it,  though  the  way  for  the  change  was  already  preparing  in 
the  Permian.  As  we  have  seen,  the  Upper  Permian,  if  classified 
by  its  plants  alone,  would  be  referred  to  the  Mesozoic  rather  than 
to  the  Palaeozoic,  and  we  are  therefore  prepared  to  learn  that  the 
Triassic  flora  is  very  similar  to  that  of  the  Upper  Permian. 

Plants. — Triassic  vegetation  is  composed  of  Ferns,  Horsetails, 
Cycads,  and  Conifers,  and  of  such  plants  were  the  Newark  coals 
of  Virginia  and  North  Carolina  and  the  Keuper  coals  of  Germany 
and  Sweden  accumulated.  The  Ferns  are  relatively  somewhat 
less  abundant  than  they  had  been  in  the  Carboniferous,  and  many 
of  them  belong  to  the  existing  tropical  family  of  the  Marattiacece. 


PLANTS  449 

T.aniopteris,  Caulopteris,  Clathropteris  (PL  VIII,  Fig.  5)  are 
among  the  most  important  genera.  In  Virginia  a  magnificent 
fern  with  very  broad  leaves,  Macrot&niopteriS)  is  the  most  abun- 
dant and  characteristic  of  the  Triassic  plants  there  found. 

The  Lycopods  have  undergone  a  great  reduction  since  the  Car- 
boniferous, though  a  few  straggling  specimens  of  Sigillaria  have 
been  found  in  the  lower 
Trias.  On  the  other 
hand,  true  Horsetails 
of  the  modern  genus 
Equisetum  now  make 
their  first  appearance, 
and  much  surpass  their 
modern  representatives 
in  size,  having  stems  of 
4  inches  in  thickness. 
Rhizomes  and  stems  of 
these  plants  are  very 
common,  and  dense 
growths  of  them,  like 
cane-brakes,  surround- 
ed the  inland  seas  and 
salt  lakes  of  the  period. 
Cycads  with  their 
stiff  leaves  abounded, 
growing,  doubtless,  on 
the  dryer  lowlands 

above  the  swamps,  most 

e     ,  ,1         •  FIG.  148.—  Voltzia  heterophylla.     (Fraas.) 

of  them  belonging  to 

such  genera  as  Pterophyllum,  Zamites,  and  Otozamites  (VIII,  6). 
This  group  of  plants  is  a  characteristic  Mesozoic  one,  and  the  era  is 
sometimes  called  the  "  Age  of  Cycads."  On  the  hills  and  uplands 
grew  dense  forests  of  Conifers,  in  appearance  like  the  Araucarians, 
which  are  found  to-day  in  South  America,  Polynesia,  and  Australia. 
Baiera,  Araucarites,  and  the  cypress-like  Voltzia  (Fig.  148),  the 
latter  much  resembling  the  Permian  Walchia,  are  common  genera. 


450  THE  TRIASSIC   PERIOD 

While  the  Triassic  flora  is  thus  different  from  that  of  the 
Palaeozoic,  it  must  have  given  to  the  landscapes  of  the  period 
much  the  same  appearance  of  graceful  and  luxuriant,  but  some- 
what gloomy  and  monotonous,  vegetation.  Probably  the  fern 
forests  of  New  Zealand  give  the  best  modern  picture  of  these 
early  Mesozoic  woodlands. 

Of  marine  plants,  the  Calcareous  Alga  should  be  mentioned. 

Among  the  animals  the  change  from  Palaeozoic  times  is  much 
more  complete  than  among  the  plants. 

Coelenterata.  —  Corals  abounded  in  the  seas,  wherever  condi- 
tions were  favourable  to  their  growth,  but  -the  Palaeozoic  Tetra- 
coralla  have  died  out,  and  their  place  is  taken  by  the  modern 
Hexacoralla^  though  the  two  groups  of  corals  approach  each  other 
so  closely  that  the  distinction  is  not  a  sharp  one. 

Echinodermata.  —  In  this  type  a  more  marked  change  has  taken 
place.  The  Cystids  and  Blastoids  have  disappeared,  and  the  Cri- 
noids  have  undergone  a  change  of  structure,  the  Palceocrinoidea 
giving  way  to  the  Neocrinoidea,  though  the  latter  occur  only  in 
small  numbers  and  in  character  rather  transitional  from  the  older 
forms  than  typical  of  the  new.  Of  the  Triassic  Crinoids  much  the 
commonest,  and  indeed  the  only  well-known  genus,  is  Encrinus, 
which  is  so  characteristic  of  the  German  Muschelkalk  ;  it  resem- 
bles more  the  Palaeozoic  than  the  later  Mesozoic  Crinoids.  Simi- 
larly, the  ancient  type  of  the  Sea-urchins,  the  Palceoechinoidea,  is 
all  but  gone,  only  a  few  persisting  through  the  Mesozoic,  while  the 
Eucchinoidea,  which  began  in  a  small  way  in  the  Carboniferous, 
now  come  to  the  front.  The  Triassic  Echinoids  all  belong  to  the 
subclass  Regulares,  the  irregular  forms  not  appearing  till  later. 

Arthropoda.  —  The  Crustacea  and  Insects  of  the  Trias  are  not 
well  known,  and  offer  no  features  of  particular  interest. 

The  Bryozoa  undergo  a  marked  change  in  the  disappearance  of 
the  ancient  Fenestella-like  genera. 

Brachiopoda.  —  One  of  the  most  important  changes  from  the 
Palaeozoic  to  the  Mesozoic  consists  in  the  great  reduction  of  the 
Brachiopods.  Even  in  the  Trias  the  reduction  is  very  marked, 
though  several  Palaeozoic  genera  have  their  latest  representatives 


MOLLUSCA 


451 


in  the  rocks  of  this  system ;  as  examples,  may  be  mentioned  Pro- 
ductus,  Athyris,  and  Cyrtina.  Koninckina  is  a  new  genus  of  the 
Spirifer  family,  which  is  confined  to  the  Trias.  The  still  existing 
genera,  Terebratula  and  Rhynchonella,  are  much  the  most  abun- 


PLATE  vill.   AMERICAN  TRIASSTC  FOSSILS 


i.  Monotis  subcircularis.  2.  Myophoria  alta.  3.  Trachyceras  Whitneyi  1/2 
4.  Arcestes  Gabbi.  5.  Clathropteris  platyphylla.  6.  Otozamites  latior.  (Figs  1-4 
after  Gabb.  Figs.  5,  6  after  Newberry.) 

dant  brachiopods  of  the  period,  and  Thecidium,  which  later  be- 
comes important,  has  its  beginning  here. 

Mollusca.  —  Almost  in  proportion  to  the  decline  of  the  brachio- 
pods is  the  rise  of  the  Pelycypoda,  or  Bivalves,  which  now  become 
far  more  varied  and  abundant  than  they  hid  been  in  the  Palaeozoic. 
Pecten,  Monotis  (VIII,  i),  Myophoria  (VIII,  2) ,  Halobia,  and  Car- 


452  THE  TRIASSIC   PERIOD 

dita,  may  be  selected  as  a  few  examples  of  the  commoner  genera. 
The  higher  forms  of  the  class,  the  Sinupalliata,  are,  however,  still 
rare.  The  Gastropoda  are  yet  in  a  transition  stage.  Several  genera, 
such  as  Murchisonia,  Loxonema,  etc.,  here  make  their  last  appear- 
ance, and  mingled  with  them  are  the  forerunners  and  earliest  repre- 
sentatives of  modern  types,  such  as  Cerithium,  Emarginula,  etc. 

The  Cephalopoda,  and  more  particularly  the  Ammonoids,  have 
already  acquired  a  wonderful  degree  of  abundance  and  variety, 
more  than  1000  species  of  the  latter  group  having  already  been 
described  from  the  Trias.  The  ancient  Nautiloid  genus  Orthoceras, 
which  ranges  from  the  uppermost  Cambrian  through  the  whole 
Palaeozoic  group,  persists  into  the  Triassic  system,  but  not  later, 
and  numerous  coiled  Nautiloids  with  angulated  and  ornamented 
shells  recall  those  of  the  Carboniferous.  Of  the  Ammonoids  some 
still  have  the  comparatively  simple  sutures  of  Goniatites,  others, 
like  Ceratites,  have  slightly  serrated  sutures,  while  in  the  upper 
Triassic  occur  some  shells  in  which  the  complexity  of  the  sutures 
is  carried  farther  than  in  any  other  known  members  of  the  group. 
Only  a  few  of  this  great  assemblage  of  genera  can  be  mentioned  ; 
especially  characteristic  of  the  Trias  are  :  Tierolites,  Trachyceras 
(VIII,  3),  Meekoceras,  Arcestes  (VIII,  4),  Ceratites,  and  Pinaco- 
ceras.  It  is  very  interesting  to  observe  that  in  the  Trias  occur, 
though  but  rarely,  certain  unusual  forms  of  Ammonoid  shells, 
which  do  not  become  important  until  the  long  subsequent  time  of 
the  Cretaceous  period.  Rhabdoceras  has  a  straight  shell,  Cochlo- 
ceras  one  that  is  coiled  in  a  high  spiral  like  a  gastropod,  and  in 
Choristoceras  the  coils  are  open.  The  Cretaceous  genera  were 
not  derived  from  these  Triassic  anticipations,  but  are  degenerate 
members  of  many  Ammonoid  families.  The  Dibranchiate  Cepha- 
lopods,  and  especially  the  characteristic  Mesozoic  group  of  the 
Belemnites,  have  their  earliest  and  most  primitive  representatives 
in  the  Triassic  genera  Atractites  and  Aulacoceras. 

The  Vertebrata  are  of  extraordinary  interest,  and  if  the  Trias 
had  yielded  only  vertebrate  fossils,  it  would  still  be  apparent  that 
great  progress  had  been  made  since  the  time  of  the  latest  known 
Palaeozoic  beds.  The  Fishes  display  this  progress  least  of  all  the 


REPTILES  453 

Vertebrates.  Shark  teeth  are  known,  but  not  skeletons.  The 
Dipnoan  Ceratodus  is  very  characteristic,  continuing  up  from  the 
Permian.  The  Crossopterygians  have  greatly  declined,  but  some 
very  large  and  curious  fishes  of  this  group,  like  Diplurus  (Fig.  149) , 
still  linger.  The  Ganoids  continue  to  be  the  dominant  fish-type, 


FIG.  149.  —  Diplurus  longicaudatus.     Newark  shales.     (Dean.) 

especially  of  the  inland  waters,  and  are  most  like  the  existing  gar- 
pikes.  Catoptems  and  Ischypterus  are  representative  American 
genera,  and  Semionotus,  Dictyopyge,  and  Lepidotus  are  nearly  allied 
European  fishes. 

The  Amphibia  reach  their  culminating  importance  in  the  Trias, 
the  Stegocephala  multiplying  and  diversifying  in  a  wonderful  fash- 
ion, and  far  surpassing  the  genera  of  the  Carboniferous  and  Per- 
mian in  size.  These  Amphibians  have  been  found  in  North 
America,  southern  Africa,  and  Europe ;  but  those  of  the  last- 
named  continent  are  much  the  best  understood,  because  best 
preserved,  the  Bunter  Sandstone  of  Germany  being  a  treasure- 
house  of  such  remains.  Mastodonsaurus,  Cyclotosanrtis,  and  Laby- 
rinthodon  are  common  European  genera,  but  there  are  many  others. 
Cheirotherium  (also  European)  is  known  only  from  its  curious 
footprints,  like  the  print  of  a  human  hand.  (See  Fig.  85, 
p.  230.) 

Reptilia.  —  It  is  in  this  class  that  we  find  the  most  remarkable 
changes ;  and  although  reptiles  are  common  in  the  Permian,  the 
abundance  and  diversity  of  the  Triassic  reptiles  are  incomparably 
greater.  Almost  all  the  orders  of  Mesozoic  reptiles  are  already 
represented  in  the  Trias,  though  often  by  comparatively  small  and 


454  THE  TRIASSIC   PERIOD 

rare  forms.  The  Triassic  reptiles  are  much  more  common  and 
better  preserved  in  Europe  than  in  America,  only  two  orders 
having  as  yet  been  found  here  ;  but  such  American  genera  as  do 
occur  show  that  there  was  no  essential  difference  between  the 
reptilian  faunas  of  the  two  continents. 

The  Rhynckocephaliat  which  are  very  near  to  the  Permian 
Proganosauria,  are  represented  by  Telerpeton  and  Hyperodapedon. 
Allied  to  the  Crocodiles  are  the  little  Aetosaurus  and  the  formi- 
dable Belodon  (Fig.  150),  the  latter  found  also  in  this  country. 
The  first  of  the  dolphin-like  Ichthyosaurs^  which  become  so 
important  in  the  Jurassic,  are  sparingly  found  in  the  Trias. 
Another  group  of  sea-dragons,  the  Plcsiosaurs,  which  attain  such 


FIG.  150.  —  Skull  of  Belodon  Kapffii,  about  ^  natural  size.     (Zittel.) 

great  development  in  Jurassic  times,  is  represented  in  the  Trias 
by  small  ancestral  forms,  Nothosaurus,  etc.  These  are  of  extraor- 
dinary interest,  as  showing  the  descent  of  the  purely  marine  Ple- 
siosaurs,  with  their  swimming  paddles,  from  terrestrial  reptiles 
which  had  feet  adapted  for  walking. 

One  of  the  most  characteristic  of  the  Mesozoic  orders  of  rep- 
tiles is  that  of  the  Dinosauria,  of  which  the  Trias  has  many 
representatives ;  but  clearly  there  were  very  many  more  than 
have  yet  been  found,  for  the  Newark  sandstones  of  the  eastern 
United  States  have  preserved  a  great  variety  of  Dinosaurian  foot- 
prints, but  very  few  bones  have  been  found  in  these  rocks.  The 
Dinosauria  were  a  much  diversified  order  of  reptiles,  adapted  for 
very  different  habits  of  life  :  some  were  herbivorous,  others  car- 
nivorous ;  some  walked  on  all  fours ;  others  were  occasionally  or 
habitually  bipedal,  and  walked  upright  after  the  manner  of  birds, 
with  which  they  have  many  structural  features  in  common.  The 


REPTILES  455 

gigantic  size  attained  by  some  of  these  creatures,  even  in  the 
Trias,  is  shown  by  the  footprints,  some  of  which  are  14  to  18 
inches  in  length.  Of  the  few  American  forms  of  which  the  bones 
have  been  found,  the  best  known  is  Anchisaurus  from  the  Con- 
necticut valley,  and  of  the  European  genera,  Zanclodon. 

The  earliest  Turtles  are  found  in  the  Triassic  of  Europe,  and 
these  first-known  members  of  the  order  are  as  typically  differen- 
tiated as  any  of  the  later  members.  No  doubt,  the  Turtles  orig- 
inated in  the  Permian,  in  some  region  as  yet  unknown,  and 
migrated  to  Europe  in  the  Trias.  The  Theromorphs,  which  we 
found  beginning  in  the  Permian,  culminated  in  the  Trias,  espe- 
cially in  southern  Africa.  Of  this  group  there  are  two  Triassic 
suborders,  the  Anomodontia  and  the  Theriodontia.  The  former 
have  cutting  jaws,  like  Turtles,  and  may  or  may  not  possess  a  pair 
of  great  tusks  in  the  upper  jaw.  Dicynodon,  a  genus  of  this  sub- 
order, has  been  found  in  southern  Africa,  India,  and  Scotland. 
The  Theriodonts  present  extraordinary  approximations  to  the 
mammals,  and  have  left  a  great  wealth  of  remains  —  some  of 
them  very  large  —  in  the  Karoo  beds  of  South  Africa,  and  less 
abundantly  in  India. 

We  thus  observe  a  notable  contrast  between  the  Triassic  reptiles 
of  Europe  and  North  America,  on  the  one  hand,  and  those  of  south 
Africa  and  India,  on  the  other.  In  the  northern  continents  the 
fauna  is  much  more  diversified,  and  consists  of  Rhynchocephalia, 
Crocodiles,  Turtles,  Ichthyosaurs,  Plesiosaurs,  a  great  variety  of 
Dinosaurs,  and  a  few  Theromorphs ;  and  associated  with  these 
are  many  great  Stegocephalous  Amphibia.  In  south  Africa  the 
reptilian  fauna  is  almost  entirely  composed  of  an  extraordinary 
variety  of  Theromorphs,  some  of  which  were  exceedingly  curious 
in  appearance,  and  in  size  ranged  from  very  large  to  very  small 
types.  India  was  the  meeting-ground  of  the  northern  and  south- 
ern faunas,  and  had  representatives  of  both. 

The  Trias  has,  as  yet,  yielded  no  Lizards,  Snakes,  or  Pterosaurs, 
'all  of  which  became  very  important  at  a  later  date.  No  birds  are 
known  from  this  period,  nor  any  reptiles  which  can  be  regarded 
as  the  ancestors  of  birds. 


456  THE  TRIASSIC   PERIOD 

Mammalia.  —  Still  another  great  advance  in  the  progress  of 
life  is  registered  in  the  first  appearance  of  the  Mammals,  which 
occurred  in  the  Trias.  Mammals  are  the  most  highly  organized 
forms  of  animals ;  but  these,  their  earliest  known  representatives, 
were  very  small  and  very  primitive,  giving  little  promise  of  being 
the  future  conquerors  of  the  world,  as  they  were  tiny  creatures 
which,  in  a  measure,  represent  the  transition  from  lower  verte- 
brates upward.  Two  American  genera,  Dromatherium  and  Micro- 
conodon,  and  one  European  genus,  Microlestes,  have  been  recoveredo 


CHAPTER   XXIX 
THE  JURASSIC  PERIOD 

THE  name  Jurassic  was  first  used  by  Brogniart  and  Humboldt, 
and  was  taken  from  the  Jura  Mountains  of  Switzerland,  where  the 
rocks  of  this  system  are  admirably  displayed.  In  Europe  the 
Jurassic  has  long  been  a  favourite  subject  of  study,  because  of 
the  marvellous  wealth  of  beautifully  preserved  fossils  which  it  con- 
tains. For  this  reason,  the  Jurassic  is  known  with  a  fulness  of 
detail,  such  as  has  been  acquired  regarding  very  few  of  the  other 
systems ;  and  no  less  than  thirty  well-defined  subdivisions  have 
been  traced  through  many  countries  of  the  Old  World.  For  this 
country  no  such  minute  subdivision  is  as  yet  possible,  and  only 
the  three  primary  divisions  of  the  system  need  be  cited. 

T         •     f  „  f  Upper  Jurassic  Series.       White  Jura,  or  Malm.1 

Jurassic  I  Oolite  \  ./T,.  JT         .    c    .         ,, 

c    *         1  I  Middle  Jurassic  Series.     Brown  Jura,  or  Dogger. 

I  Lower  Jurassic  or  Liassic  Series. 

DISTRIBUTION  OF  JURASSIC  ROCKS 

American.  —  No  undoubted  Jurassic  strata  occur  in  the  Atlantic 
border  of  the  United  States,  though  by  some  authorities  the  upper- 
most part  of  the  estuarine  Trias  {Newark  Series)  is  referred  to 
this  system,  and  by  others  the  Potomac  Series  of  the  Cretaceous 
is  regarded  as  Jurassic.  Whether  or  not  these  references  be  cor- 
rect, no  marine  Jura  is  known  on  the  Atlantic  slope  of  North 
America  except  in  Mexico  :  in  the  latter  country  are  some  marine 
beds  which  probably  belong  to  this  system.  In  eastern  North 
America  the  Jura  was  a  time  of  great  denudation,  when  the  high 

1  The  Malm  is  not  quite  the  same  as  the  White  Jura,  but  includes  some  of  the 
Brown,  the  remainder  of  which  corresponds  to  the  Dogger. 

457 


458  THE  JURASSIC   PERIOD 

ranges  of  the  Appalachian  Mountains  were  much  wasted  away, 
and  the  newly  upheaved,  tilted,  and  faulted  beds  of  the  Trias  were 
deeply  eroded. 

In  the  interior  region  of  the  continent  the  course  of  events  was 
different.  The  Colorado  island  became  joined  to  the  mainland, 
forming  a  peninsula.  In  the  great  area  from  the  Uinta  Moun- 
tains southward  to  New  Mexico  and  Arizona,  and  extending  from 
the  Colorado  peninsula  westward  to  the  Great  Basin  land,  was  an 
interior  sea  or  salt  lake,  in  which  were  laid  down  great  thicknesses 
of  barren  sandstone,  without  fossils.  These  beds  are  placed  in 
the  Jurassic  entirely  upon  stratigraphic  grounds,  and  whether  they 
represent  the  whole  Jura,  or  only  a  part,  and  if  so,  what  part,  are 
questions  that  cannot  at  present  be  answered.  West  of  the  Great 
Basin  land,  the  Lias  is  found  in  California  and  Oregon,  but  not  as 
yet  in  British  Columbia,  and  recurs  in  several  of  the  Arctic  islands. 
The  faunal  relations  of  this  Pacific  border  Lias  are  with  that  of 
central  Europe. 

The  Middle  Jurassic  of  North  America  is  still  very  little  known, 
but  appears  to  have  much  the  same  distribution  as  the  Lias. 

In  the  upper  Jura  some  extensive  geographical  changes  occurred. 
Early  in  this  epoch  a  depression  was  formed  in  the  northern  inte- 
rior region,  allowing  the  waters  of  the  ocean  to  flow  in  and  estab- 
lish a  gulf  over  portions  of  what  had  been  the  Triassic  inland  sea. 
The  boundaries  of  this  great  gulf  are  not  yet  fully  known,  but  it 
extended  over  the  northern  portion  of  Utah,  where  the  Wasatch 
and  Uinta  Mountains  now  are,  and  covered  most  of  Wyoming, 
as  far  east  as  the  Black  Hills,  together  with  southern  Montana. 
Although  Jurassic  exposures  have  not  been  found  in  Canada  east 
of  the  Cordillera,  yet  the  fossils  of  this  gulf  show  that  its  waters 
were  completely  separated  from  those  of  California,  which  still  re- 
tain their  European  connection,  and  were  probably  derived  from  a 
transgression  from  the  north  or  northwest.  The  sediments  depos- 
ited in  the  gulf  are  largely  limestones,  though  with  much  clay  shale 
and  marl,  while  the  presence  of  gypsum  points  to  the  existence 
of  lagoons.  The  beds  attain  their  maximum  thickness  of  about 
1800  feet  in  the  Wasatch  Mountains,  which  then  formed  the  east- 


CLOSE  OF  THE   PERIOD  459 

ern  coast  of  the  Great  Basin  land.  These  strata  have  yielded 
many  marine  fossils,  but  the  fossils  are  very  scanty  as  compared 
with  those  of  the  beds  formed  in  the  open  sea. 

In  the  later  part  of  the  Jura,  or  upper  Malm,  still  further  changes 
were  produced.  In  the  Old  World  this  was  the  time  of  a  vast 
transgression  of  the  sea,  but  in  North  America  the  land  was  rising, 
gradually  drying  up  the  great  northern  gulf,  interrupting  the  con- 
nection with  central  European  waters  which  had  so  long  been 
shown  by  the  fossils  of  the  Pacific  border,  and  bringing  in  a 
transgression  of  the  Arctic  Sea,  which  extended  its  waters  all  down 
the  western  coast  of  North  America  as  far  as  Mexico,  but  was  sep- 
arated in  some  way  from  the  sea  which  washed  the  west  coast  of 
South  America.  Upper  Jurassic  strata  are  found  in  British  Colum- 
bia and  California,  where  they  form  an  enormously  thick  series, 
principally  of  slates,  with  interstratified  beds  of  diabase  tuffs,  which 
show  that  volcanic  activity  prevailed  along  the  shores,  or  in  the 
bed  of  the  sea.  In  the  Sierra  Nevada  these  slates  are  traversed 
by  numerous  gold-bearing  quartz  veins. 

The  close  of  the  Jurassic  was  accompanied  in  North  America 
by  a  time  of  upheaval  and  mountain  making  along  the  western 
side  of  the  continent,  corresponding  to  the  Appalachian  revolution 
which  had  occurred  at  the  close  of  the  Palaeozoic  era  along  the 
eastern  side.  The  Sierra  Nevada  had  for  long  ages  been  a  sinking 
geosynclinal  trough,  in  which  great  thicknesses  of  sediment  had 
accumulated ;  now,  at  length,  it  yielded  to  the  force  of  lateral 
compression,  which  ridged  the  strata  up  into  great  folds.  By  this 
movement  the  Pacific  coast-line  was  transferred  from  the  eastern  to 
the  western  side  of  the  mountains,  and  probably  the  Coast  Range 
was  upheaved,  forming  a  chain  of  islands.  Little  is  known  con- 
cerning this  primary  condition  of  the  Sierra  Nevada,  which  had 
not  yet  become  separated  from  the  Great  Basin  by  faults,  the 
present  mountains  being  due  to  long  subsequent  movements.  In 
the  interior  of  the  continent  also  these  movements  brought  about 
great  changes,  though  it  is  not  probable  that  the  geographical  modi- 
fications of  the  interior  all  occurred  at  the  same  time,  or  that  they 
were  all  connected  with  the  upheaval  of  the  Sierra  Nevada.  We 


460  THE  JURASSIC   PERIOD 

have  already  seen  that  marked  changes  were  in  progress  during 
the  upper  Malm.  The  result  of  the  diastrophic  movements  in  the 
interior  during  the  latter  part,  and  at  the  close  of  the  Jurassic 
period,  was  to  drain  the  inland  seas  and  the  great  northern  gulf, 
converting  nearly  the  whole  region  into  land. 

Foreign.  — The  greater  part  of  South  America  was  above  the  sea 
during  the  Jurassic  period,  as  it  had  been  in  the  Trias.  Marine 
deposits  of  the  former  period  are  found  only  along  the  western 
border  of  the  continent,  where  they  extend  from  5°  to  35°  S.  lat. 
Throughout  the  Jurassic  the  sea  which  covered  this  western  coast 
retained  its  faunal  connection  with  the  central  European  region, 
and  even  the  minuter  divisions,  the  substages  and  zones  of  the 
European  Jura,  are  applicable  to  the  classification  of  the  South 
American  beds.  In  what  manner  this  southern  area  became 
separated  from  the  Pacific  border  of  North  America  in  the  upper 
Malm,  it  is  difficult  to  conjecture. 

In  Europe  the  Jurassic  rocks  are  magnificently  displayed,  but 
they  differ  much  both  in  thickness  and  in  character  as  they  are 
traced  from  one  country  to  another,  which  results  from  more 
frequent  and  more  localized  changes  of  level  than  had  occurred 
during  the  Palaeozoic. 

The  Lias  has  -a  much  more  restricted  extension  than  the  later 
Jurassic  stages.  At  the  end  of  the  Triassic  had  begun  a  trans- 
gression of  the  sea,  which  flooded  many  of  the  inland  basins  and 
salt  lakes,  and  the  same  transgression  continued  into  the  Lias, 
proceeding  northward  from  the  Mediterranean,  and  covering  large 
areas  in  central  and  southern  Europe,  as  well  as  a  belt  across 
England,  but  not  extending  to  Russia.  By  far  the  greater  part  of 
the  Eurasian  land  mass  was  above  the  sea,  and  fresh-water  lakes 
extended  across  Siberia,  while  in  China  wide-spread  deposits  of 
Liassic  coal  were  accumulated. 

In  the  latter  part  of  the  middle  Jura  the  transgression  of  the 
ocean  was  renewed,  and  this  time  on  a  vastly  larger  scale,  cutting 
the  continents  by  seas  and  straits,  invading  great  areas  that  had 
long  been  land,  and  covering  the  larger  part  of  Europe  and  Asia. 
This  is  one  of  the  greatest  transgressions  of  the  sea  in  all  recorded 


FOREIGN  461 

geological  history,  but  it  did  not  greatly  affect  North  America. 
Central  and  northern  Russia  were  invaded  by  an  extension  of  the 
sea  from  the  north,  and  in  this  Russian  sea  was  developed  a  highly 
characteristic  fauna.  Strata  distinguished  by  the  Russian  fauna 
extend  through  Siberia,  Spitzbergen,  Nova  Zembla,  Alaska,  the 
Black  Hills  of  South  Dakota,  and  the  uppermost  Jurassic  of  Cali- 
fornia and  Mexico.  In  peninsular  India  the  Jura  is  represented 
by  the  upper  division  of  the  Gondwana  system,  which,  as  before, 
was  laid  down  in  an  inland  sea.  The  continental  mass  to  which 
India  then  belonged  was  cut  off  from  Asia  by  a  strait  or  sea  which 
covered  the  site  of  the  Himalayas,  but  it  may  have  been  con- 
nected with  Australia  by  way  of  the  Malay  islands,  or,  more  likely, 
with  South  Africa.  The  great  Jurassic  transgression  submerged 
considerable  areas  of  northern  India,  as  it  also  covered  narrow 
areas  along  the  eastern  and  western  coasts  of  Australia.  Much  of 
Madagascar  was  under  water,  but  the  southern  portion  is  believed 
to  have  formed  part  of  the  narrow  land  which  extended  from 
South  Africa  to  India.  Some  of  eastern  Africa  was  covered  by  a 
bay  of  the  Indian  Ocean,  but  no  marine  Jurassic  has  been  found  in 
the  southern  or  western  portions  of  that  continent. 

The  very  striking  faunal  differences  which  obtain  between  dif- 
ferent regions  have  led  certain  observers,  especially  the  late  Pro- 
fessor Neumayr  of  Vienna,  to  the  conclusion  that  climatic  zones 
had  already  been  established  in  Jurassic  times,  —  Boreal,  central 
European,  and  Alpine  or  Equatorial  zones,  with  corresponding 
ones  in  the  Southern  Hemisphere.  This  conclusion  as  to  climatic 
belts  is,  however,  a  very  doubtful  one,  and  is  in  conflict  with  the 
distribution  of  the  several  geographical  faunas,  for  the  central 
European  fauna  is  found  in  equatorial  Africa,  and  the  supposed 
equatorial  fauna  occurs  in  the  Andes  20°  of  latitude  south  of  its 
proper  boundary.  It  is  much  more  likely  that  the  marked  faunal 
differences  are  due  to  varying  facies,  depth  of  water,  character  of 
bottom,  etc.,  and  even  more  to  the  partly  isolated  sea-basins  and 
the  changing  connections  which  were  established  between  them. 
There  is  no  cogent  evidence  to  show  that  the  Jurassic  climate  was 
less  mild,  equable,  and  uniform  than  that  of  preceding  periods, 


462  THE  JURASSIC   PERIOD 

JURASSIC  LIFE 

The  life  of  the  Jurassic  has  been  preserved  in  wonderful  fulness 
and  variety;  but  with  comparatively  few  exceptions,  our  knowledge 
of  it  has  been  principally  derived  from  Europe,  where  a  host  of 
eminent  geologists  have  long  studied  the  great  wealth  of  material. 
The  contrast  between  North  America  and  Europe  in  regard  to  the 
relative  abundance  of  Jurassic  marine  fossils  is  seen  from  the  fact 
that  while  in  Great  Britain  alone  more  than  4000  species  have 
been  described,  in  America  hardly  one-tenth  of  that  number  has 
so  far  been  found. 

Plants.  —  The  flora  of  the  Jurassic  differs  little,  on  the  whole, 
from  that  of  the  Trias,  and  is  made  up  of  Ferns,  Horsetails, 
Cycads,  and  Conifers.  Tree  ferns  flourished  in  northern  Europe 
in  great  variety.  The  Cycads  attain  their  culmination  of  abundance 
and  diversity  in  this  period,  no  less  than  forty  species  occurring 
in  a  single  horizon  of  the  English  Upper  Jura ;  some  of  them,  like 
Palaozamia,  have  leaves  exceeding  one  foot  in  length.  The  Coni- 
fers are  of  somewhat  more  modern  aspect  than  those  of  the  Trias, 
and,  from  their  resemblance  to  genera  which  are  still  extant,  have 
received  such  names  as  Thujites,  Taxites^  Cupres sites,  Pinites.  etc. 
The  Araucarian  pines  still  flourished  in  Europe.  Monocotyledons 
have  been  reported  from  the  Jurassic,  but  the  evidence  for  their 
existence  is  very  doubtful. 

Foraminifera  are  found  in  great  numbers  and  variety  in  the  soft 
Jurassic  clays,  many  of  them  belonging  to  genera  which  still 
abound  in  the  modern  seas.  It  must  not  be  supposed  that  these 
organisms  first  became  so  abundant  in  Jurassic  times ;  it  is  merely 
that  the  conditions  for  the  preservation  of  these  microscopic  and 
exquisite  shells  had  not  been  so  favourable  before. 

Radiolaria.  —  The  beautiful  siliceous  tests  of  the  Radiolarians 
are  also  found  in  multitudes.  In  the  Alps  occur  whole  strata  of 
red  flints  and  jaspery  slates,  which  are  composed  almost  entirely 
of  these  tests. 

Spongida.  —  Sponges  are  found  in  wonderful  profusion  and 
diversity  and  in  such  perfect  preservation  that  every  detail  of  their 


CCELENTERATA 


463 


beautiful  structure  may  be  made  out  with  the  microscope.  In 
some  localities  these  sponges  are  heaped  up  in  such  masses  that 
they  fill  the  strata,  while  other  localities  of  the  same  horizon  are 
entirely  free  from  them. 


PLATE  IX.   AMERICAN  JURASSIC  FOSSILS 

i.  Segment  of  stem  of  Pentacrinus  asteriscus,  4/1.  2.  Gryphaea  nebrascensis. 
3.  Quenstedioceras  cordiforme,  2/3.  4.  Belemnites  densus,  2/3.  5.  Trigonia  ameri- 
cana.  (After  Meek  and  Hayden.) 

Coelenterata.  —  Corals  abound,  especially  in  the  Upper  Jurassic 
of  central  Europe.  The  Anthozoan  Corals  all  belong  to  the  mod- 
ern Hexacoralla,  in  decided  contrast  to  the  Tetracoralla  of  the 
Palaeozoic  seas.  Isastrcea,  Montlivaultia,  and  Thecosmilia  are  the 
dominant  genera. 


464  THE  JURASSIC   PERIOD 

The  Echinodermata,  especially  the  Crinoids  and  Sea-urchins, 
are  of  great  importance.  The  Crinoids  are  vastly  more  abundant 
than  they  had  been  in  the  Trias,  and  although  the  number  of 
genera  and  species  is  not  at  all  comparable  to  the  great  assem- 
blage of  Carboniferous  time,  yet  for  profusion  and  size  of  indi- 
viduals the  Jurassic  has  never  been  surpassed.  Especially  charac- 
teristic are  the  superb  species  of  Pentacrinus  (PI.  IX,  Fig.  i),  a 
genus  which  still  exists  in  the  West  Indian  seas.  Other  common 
genera  are  Apiocrinus  and  Eugeniacrinus.  These  genera  all 
belong  to  the  NeocrinoideH,  which  have  a  very  different  type  of 
structure  from  the  Palaeozoic  forms,  but,  like  nearly  all  the  latter, 
they  were  attached  to  the  sea-bottom  by  their  long  stems.  In  the 
Jurassic  first  appear  the  free-swimming  Crinoids,  like  Comatula, 
the  commonest  of  modern  genera.  These  animals  possess  a  stem 
only  in  their  early  stages  of  development ;  subsequently  they 
become  detached  and  free.  Star-fishes  and  Brittle  Stars  are  not 
very  common,  but  have  attained  a  completely  modern  structure. 

The  Echinoids  have  undergone  a  wonderful  expansion  and 
diversification  by  the  time  of  the  Middle  Jurassic.  In  the  Lias,  as 
in  the  Trias,  we  find  only  the  regular,  radially  symmetrical  sea- 
urchins,  with  mouth  and  anus  at  the  opposite  poles  of  the  shell, 
but  in  the  middle  and  upper  Jura  appear  the  irregular  Spatangoids 
and  Clypeastroids.  In  these  the  shell  is  bilaterally  symmetrical, 
rather  than  radially  so,  the  anus,  and  even  the  mouth,  losing  their 
polar  positions,  and  the  shape  of  the  ambulacral  areas  being 
greatly  changed.  This  is  another  instance  of  the  attainment  of 
modern  structure  which  so  many  of  the  Mesozoic  Invertebrates 
display. 

Arthropoda.  —  The  Crustacea  are  not  found  in  very  many  locali- 
ties, but  places  like  the  famous  lithographic  limestones  of  Solen- 
hofen  in  Bavaria,  where  the  conditions  of  preservation  were 
favourable,  show  that  this  group  was  very  abundant  and  far  ad- 
vanced in  the  Jurassic  seas.  The  long-tailed  (macrurous)  Deca- 
pods (of  which  the  lobster  is  a  familiar  example)  are  in  the 
ascendant  and  are  represented  by  many  genera,  several  of  which 
still  exist.  The  Crabs,  or  short-tailed'  Decapods,  which  are  now 


MOLLUSC  A  465 

so  very  common,  make  their  first  known  appearance  in  the  Jurassic, 
but  they  were  still  rare,  and  connecting  links  between  the  long- 
tailed  and  short-tailed  series  were  more  abundant.  Isopods  and 
Stomatopods  also  abounded. 

The  Limuloids  are  reduced  to  the  single  genus  Limulns,  which 
then  occurred  in  the  European  seas,  while  the  living  horseshoe 
crabs  of  that  genus  are  found  only  on  the  east  coast  of  the  United 
States  and  in  the  Molucca  Islands. 

Spiders  and  Centipedes  have  not  yet  been  found, — ;  another 
illustration  of  the  imperfection  of  the  geological  record.  There 
can  be  no  doubt  that  these  animals  existed  in  Jurassic  times,  for 
we  find  them  both  before  and  after  that  period. 

Insects,  on  the  other  hand,  are  found  in  multitudes  in  certain 
localities,  and  display  a  great  advance  in  the  number  of  types  over 
any  of  the  Palaeozoic  periods.  The  Orthopters  and  Neuropters 
which  we  found  in  the  Palaeozoic  are  enriched  by  many  new 
forms,  such  as  grasshoppers  and  dragon-flies,  while  beetles  ( Cole- 
opterd]  become  very  abundant.  The  Hymenopters  (ants,  bees, 
wasps,  etc.)  and  the  Dipters  (flies)  date  from  the  Jurassic,  and 
Lepidopters  (butterflies)  have  also  been  reported,  though  doubt- 
fully. As  the  latter  insects  are  dependent  upon  a  flowering  vege- 
tation, definite  proof  of  their  presence  in  the  Jura  will  establish 
the  existence  of  the  Angiosperms. 

Brachiopoda.  —  These  shells  are  still  common  in  the  Jura,  but 
they  are  simply  a  profusion  of  individuals  belonging  to  a  few 
genera,  most  of  which  persist  in  our  recent  seas.  Terebratula, 
Waldheimia,  and  Rhynchonella  are  much  the  most  important 
genera,  and  the  last  stragglers  of  the  long-lived  Palaeozoic  Spiri- 
ferina  are  here  found. 

Mollusca.  —  The  Bivalves,  which  had  already  become  such  im- 
portant elements  of  the  Triassic  fauna,  greatly  increase  in  the  Jura, 
their  shells  forming  great  banks  and  strata.  Many  of  the  genera 
are  still  living,  and  only  a  few  of  the  more  abundant  ones  can  be 
mentioned  here.  Oysters  like  Gryphcea  (IX,  2),  Exogyra,  and 
Ostrea  itself  are  common.  Trigonia  (Fig.  151  and  PL  IX,  Fig.  5) 
is  especially  characteristic  of  the  Jura,  but  a  few  representatives 

2  H 


466  THE  JURASSIC   PERIOD 

of  that  genus  have  persisted  to  the  present  time  and  are  found  in 
the  Australian  seas.  Diceras  and  Pholadomya  are  likewise  com- 
mon genera,  and  there  are  very  many  others.  Among  the  Gas- 
tropoda the  most  significant  change  lies  in  the  importance  which 
the  siphon-mouthed  shells  (Siphonostomata]  now  for  the  first  time 
assume ;  examples  of  this  group  are  Nerinia,  Alaria,  Purpurina, 
etc.  Of  the  shells  with  entire  mouths  (Holostomata)  the  ancient 


FIG.  151.  —  Slab  of  Trigonia  clavellafa,  from  the  English  Jura. 

Palaeozoic  genus  Plenrotomaria  is  as  abundant  as  ever,  not  begin- 
ning to  decline  until  the  Cretaceous  period. 

The  Cephalopods  are  at  the  very  height  of  their  culmination, 
and  are  present  in  an  astonishing  profusion  and  diversity,  filling 
whole  strata  with  their  heaped-up  shells.  The  Nautiloids  differ 
from  those  of  the  Trias  in  their  smoother  and  more  involute  shells. 
The  Ammonoids  do  not  display  so  many  types  of  shell  structure 
as  we  have  found  in  the  Trias,  and  the  genera  are  mostly  different 
from  those  of  the  latter  period  ;  but  in  number  of  distinct  species 
the  Jura  much  surpasses  the  other  Mesozoic  periods.  Phylloceras 
and  Lytoceras  continue  on  from  the  Trias,  but  the  most  abundant, 


FISHES  467 

characteristic,  and  widely  spread  genera  are  new.  Of  these  may 
be  mentioned  :  Arietites,  sEgoceras,  Harpoceras,  Stephanoceras, 
Perisphinctites,  and  many  others,  each  with  large  numbers  of  spe- 
cies. The  Belemnites,  which  were  introduced  in  a  small  way  in 
the  Trias,  in  the  Jurassic  blossom  out  into  an  incredible  number 
of  forms,  exceeding  even  the  Ammonites  in  abundance  of  indi- 
viduals, if  not  of  species.  These  extinct  Cephalopods  belonged 
to  the  Dibranchiata,  as  do  all  the  living  forms  except  the  pearly 
Nautilus;  they  in  some  measure  serve  to  connect  the  extinct 
genera  having  external  shells  with  the  existing  naked  squids  and 
cuttle-fishes,  which  have  only  rudimentary  internal  shells,  the  pen 
or  cuttle-bone.  The  Belemnites  have  a  straight,  conical,  cham- 
bered shell,  called  the  phragmocone,  which  ends  above  in  a  broad, 
thin  plate.  The  phragmocone  was  partly  external  to  the  animal, 
and  its  lower,  pointed  end  was  inserted  into  a  dart-  or  club-shaped 
body  called  the  guard  or  rostrum  (PI.  IX,  Fig.  4),  which  is  com- 
posed of  dense,  fibrous,  crystalline  calcite.  Usually  only  the  guard 
is  preserved  in  the  fossil  state,  and  specimens  are  so  common  that 
they  have  attracted  popular  interest  and  bear  the  folk-name  of 
"  thunderbolts."  In  a  few  instances  the  animal  has  been  preserved 
almost  entire,  so  that  the  structure  is  well  understood. 

Vertebrata. — The  Fishes  have  advanced  much  beyond  those 
of  the  Trias.  The  Sharks  have  attained  practically  their  modern 
condition,  and  the  broad,  flattened  Rays  are  a  new  type  of  the 
order.  The  Chimaroids  were  much  more  numerous  and  rela- 
tively important  than  they  are  at  present,  when  only  a  few  are 
left.  The  Dipnoans  have  become  very  scarce  and  are  hardly  rep- 
resented in  the  Northern  Hemisphere,  save  for  the  persistence  of 
Ceratodus.  The  Crossopterygians  are  greatly  reduced,  though  a 
few  exceedingly  curious  forms,  like  Undina,  still  linger.  Of  the 
Teleostome  fishes  the  Ganoids  are  still  the  dominant  type,  as  they 
had  been  since  the  Devonian.  Some  of  these  Jurassic  forms  are 
evidently  the  forerunners  of  the  Sturgeons,  but  most  of  them  re- 
semble the  Gar-pike  of  our  Western  rivers  (Lepidosttus),  and  are 
covered  with  a  heavy  armour  of  thick,  shining,  rhomboidal  scales. 
Many  of  these  Ganoids  are  of  small  or  moderate  size,  such  as 


468 


THE  JURASSIC   PERIOD. 


Dapedius  (Fig.  152)  and  Aspidorhynchus  (Fig.  153),  while  others, 
like  the  superb  Lepidotus,  were  very  large,  evidently  the  kings  of 
their  time  and  race.  Some  of  the  Jurassic  fishes  approximate  the 
Teleosts  so  closely  that  it  seems  arbitrary  to  call  them  Ganoids. 
Catunts,  Leptolepis,  Hypsocormus  (Fig.  154),  and  Megalurus  are 
much  like  what  the  ancestral  Teleosts  must  have  been. 


FIG.  152. —  Dapedius  politus.     (Smith  Woodward.) 


FlG.  153.  —  Aspidorhynchus  acutirostris.     (Smith  Woodward.) 


FlG.  154.  —  Hypsocormus  insignis.     (Smith  Woodward.) 


ICHTHYOSAURS 


469 


No  Amphibia  are  certainly  known  from  the  Jurassic. 

The  Reptiles  have  attained  a  higher  and  more  diversified  plane 
of  existence  than  in  the  Trias.  Most  of  the  Triassic  genera  and 
one  entire  reptilian  order,  the  Theromorphs,  have  become  extinct, 
but  new  and  more  advanced  forms  come  in  to  take  their  places. 
The  Rhynchocephalians  continue  and  the  first  of  the  true  Lizards 
(Lacertilia)  appear.  Turtles  abound,  having  grown  much  more 
numerous  than  in  the  Trias.  The  Ichthyosauria  are  a  highly 
characteristic  Jurassic  group ;  for  though  they  are  found  in  both 
the  Trias  and  the  Cretaceous,  the  Jura,  and  especially  the  Lias,  is 
the  time  of  their  principal  expansion.  Certain  localities  in  the 
Lias  of  England  and  Germany  have  yielded  an  incredible  number 


FIG.  155.  —  Restoration  of  Ichthyosaurus  quadriscissus.     (E.  Fraas.) 

of  skeletons,  and  some  of  the  specimens  have  preserved  the  im- 
pressions of  the  outline  of  the  body  and  limbs,  showing  recogniz- 
ably the  nature  of  the  skin.  These  reptiles  were  entirely  marine 
in  their  habits  and  preyed  upon  fishes,  and  their  limbs  were  con- 
verted into  swimming  paddles ;  there  are  several  small  dorsal  fins 
and  a  large  tail-fin,  the  principal  organ  of  propulsion  (see  Fig. 
155).  The  muzzle  is  drawn  out  into  an  elongate  slender  snout, 
armed  with  numerous  sharp  teeth,  which  were  set  in  a  continuous 
groove,  not  in  separate  sockets.  The  eye  is  very  large  and  pro- 
tected by  a  number  of  bony  plates,  which  are  often  preserved  in 
the  fossil  state.  The  neck  is  very  short  and  hardly  distinguished 
from  the  porpoise-like  body.  The  skin  was  smooth,  having  neither 
horny  scales  nor  bony  scutes,  which  was  of  advantage  in  lessening 
the  friction  of  the  water.  In  length,  these  reptiles  sometimes  ex- 


470 


THE  JURASSIC  PERIOD 


ceeded  25  feet.  Baptanodon,  found  in  Wyoming,  is  an  Ichthyo- 
saur  without  teeth  and  must  have  fed  upon  small  and  soft  marine 
invertebrates,  as  do  the  toothless  whales. 

Another  group  of  carnivorous  marine  reptiles  is  that  of  the 

Plesiosauria,  which  began  in  the 
Trias  and  culminated  in  the  Jura, 
and  which  forms  a  curious  contrast 
to  the  Ichthyosaurs.  In  the  typical 
genus  Plesiosaurus  (Fig.  156)  the 
head  is  relatively  very  small,  and  the 
jaws  are  provided  with  large,  sharp 
teeth,  set  in  distinct  sockets.  The 
neck  is  exceedingly  long,  slender, 
and  serpent-like,  and  marked  off 
distinctly  from  the  small  body.  The 
swimming  paddles  are  much  larger 
than  in  the  Ichthyosaurs  and  proba- 
bly had  more  to  do  with  locomotion  ; 
the  skeleton  of  the  paddle  departs 
much  less  widely  from  the  structure 
of  a  terrestrial  reptile's  foot  than 
does  that  of  an  Ichthyosaur.  With 
their  long  necks,  the  Plesiosaurs 
could  lie  motionless  far  below  the 
surface,  occasionally  raising  their 
heads  above  the  water  to  breathe, 
or  darting  them  to  the  bottom  after 
their  prey,  which  consisted  chiefly 
of  fish.  The  Jurassic  species  of 
Plesiosaurus  do  not  much  exceed  a 
length  of  20  feet,  but  Pliosaurus  of 
the  same  group  was  gigantic,  a  single  paddle  sometimes  measuring 
6  feet  in  length ;  the  reptiles  of  the  latter  genus  had,  however, 
proportionately  larger  heads  and  shorter  necks. 

The  seas  and  rivers  of  Jurassic  times  were  swarming  with  Croco- 
diles, Teleosaurus  being  the  characteristic  genus  of  the  period.     In 


PTEROSAURS 


471 


appearance  these  reptiles  much  resembled  the  modern  Gavial  of 
India  and  had  a  similar  elongate  and  slender  snout.  The  fore 
legs  were  much  smaller  than  the  hind,  and  these  animals  were 
doubtless  of  more  exclusively  aquatic  habit  than  the  crocodiles 
and  alligators  of  the  present  day. 

The  Dinosauria  have  become  much  larger,  more  numerous  and 
diversified  than  they  had  been  in  the  Trias,  though,  as  the  foot- 
prints in  the  Newark  sandstones  teach  us,  only  a  small  fraction  of 
the  Triassic  Dinosaurs  has  yet  been  recovered.  Making  all  due 
allowance  for  this,  it  seems,  nevertheless,  to  be  true  that  the  group 
had  made  notable  progress  in  the  Jurassic.  The  group  of  Dino- 
sauria is  a  heterogeneous  one,  comprising  reptiles  of  very  different 
size,  appearance,  structure,  and  habits  of  life.  Some  were  heavy, 
slow-moving  quadrupeds,  having  fore  and  hind  legs  of  not  very 
unequal  length,  with  hoof-like  toes,  and  usually  with  very  small 
heads.  Dinosaurs  of  this  type  were  mostly  plant-feeders  and  had 
rows  of  grinding  teeth  adapted  for  such  a  diet.  Cetiosaurus  is  an 
example  of  this  kind  of  Dinosaur,  which  attained  a  length  of  40 
feet.  Sceliddsaurus  is  another  herbivorous  reptile,  but  with  such 
short  fore  legs  that  the  gait  must  have  been  bipedal,  or  else  the 
back  must  have  been  arched  upward  very  strongly  to  the  hind 
quarters.  This  animal,  and  its  ally,  Omosauriis,  were  provided 
with  an  armour  of  bony  plates  and  spines  covering  the  back  and 
tail.  Megalosaurus,  on  the  other  hand,  was  a  gigantic  carnivorous 
Dinosaur,  having  terrible,  sharp-pointed  teeth,  while  the  toes  were 
armed  with  sharp,  curved  claws.  These  creatures  walked  upon 
their  elongated  hind  legs  and  were  the  most  formidable  beasts  of 
prey  that  scourged  the  Jurassic  lands.  Not  all  of  the  Jurassic 
Dinosaurs  were  gigantic ;  very  small  ones  also  ranged  through  the 
forests  or  may  even  have  been  arboreal  in  their  habits.  Compso- 
gnathus,  for  example,  was  a  bipedal,  carnivorous  Dinosaur  hardly 
larger  than  a  house  cat. 

Another  very  remarkable  order  of  reptiles,  the  Pterosauria, 
appears  for  the  first  time  in  the  Jurassic  (Fig.  157).  These  ani- 
mals were  provided  with  wings,  and  were  true  fliers,  thus  realizing 
the  old  myth  of  flying  dragons.  The  head  is  relatively  large,  but 


472  THE  JURASSIC   PERIOD 

very  lightly  constructed,  and  set  at  right  angles  with  the  neck,  as 
in  birds.  In  the  Jurassic  species  the  jaws  are  more  or  less  com- 
pletely armed  with  teeth,  which  by  their  form  show  the  carnivo- 
rous propensities  of  the  animal.  The  joints  of  the  external  or 
little  finger  of  the  hand  are  much  thickened  and  elongated,  this 
finger  being  longer  than  the  body  and  legs  together.  A  mem- 
brane, or  patagium,  was  stretched  between  the  elongate  finger 
on  one  side,  and  the  body  and  leg  on  the  other,  thus  forming  the 


wing,  which  rather  resembled  the 
wing  of  the  bat  than  that  of  a  bird, 
though  differing  from  the  former  in  being 
supported  by  one  finger  instead  of  four.  A 
few  exceptionally  well  preserved  specimens 
found  in  the  Solenhofen  limestones  have 
retained  the  clearly-marked  impressions  of 
these  wing  membranes.  The  legs,  like  those 
of  bats,  were  small  and  weak,  and  the  tail 
was  very  short  in  some  species,  very  long 
in  others.  Some,  at  least,  of  the  latter  had 
a  membranous,  oar-like  expansion  at  the 

tip  of  the  tail.     That  the   Pterosaurs  had 
FIG.  157. -Restoration    the  power  of  true  fljght   and  did  nQt  merely 
of   Pterosaurian,    Rham-  '  ...         .        _   . 

phorhynchus.  (Zittel.)  take  great  leaps  like  the  flying  squirrels, 
is  shown  by  the  hollow,  pneumatic  bones 
(like  those  of  birds),  and  by  the  keel  on  the  breast-bone  for  the 
attachment  of  the  great  muscles  of  flight.  This  keel  is  found  in 
both  birds  and  bats.  The  skin  was  naked,  having  neither  scales 
nor  feathers.  The  Jurassic  Pterosaurs  were  small,  the  spread  of 
wings  not  exceeding  3  feet. 

Birds.  —  One  of  the  most  remarkable  advances  which  Jurassic 
life  has  to  show  consists  in  the  first  appearance  of  the  birds.  As 
yet,  only  a  single  kind  of  Jurassic  bird  has  been  found,  and  that 
in  the  Solenhofen  limestones.  This,  the  most  ancient  known  bird, 


MAMMALS 


473 


is  called  Archaopteryx  (Fig.  158),  and  has  many  points  of  resem- 
blance to  the  reptiles,  and  many  characters  which  recur  only  in 
the  embryos  of  modern  birds.  The  peculiarities  which  strike  one 
at  the  first  glance  are  the  head  and  tail;  there  was  no  horny 
beak,  but  the  jaws  are  set  with  a  row  of  small  teeth,  while  the  tail 
is  very  long,  composed  of  separate  vertebrae,  and  with  a  pair  of 
quill  feathers  attached  to  each  joint.  The  wing  is  constructed  on 
the  same  plan  as  that  of  a  modern  bird,  but  is  decidedly  more 
primitive.  The  four  fingers  are 
all  free  (in  recent  birds  two  of 
them  are  fused  together)  ;  they 
have  the  same  number  of  joints 
as  in  the  lizards,  and  are  all 
provided  with  claws.  The  plu- 
mage is  thoroughly  bird-like  in 
character,  but  is  peculiar  in  the 
presence  of  quill  feathers  on  the 
legs.  This  very  extraordinary 
creature  was,  then,  a  true  bird, 
but  had  retained  many  features 
of  its  reptilian  ancestry,  and 
shows  us  that  those  ancestors 
have  still  to  be  sought  in  the 
Trias  or  even  the  Permian. 
Mammalia.  —  The  mammals 


of  the  Jurassic  are  still  very  rare 


FIG.  158.— Restoration  of  Archaopteryx 
macrura.     (Andreae.) 

and  imperfectly  known,  and  have  been  found  in  only  a  few  places. 
How  many  mammalian  genera  should  be  referred  to  the  Jurassic 
will  depend  upon  where  the  somewhat  arbitrary  line  is  drawn, 
which  separates  that  system  from  the  Cretaceous.  Excluding  the 
transition  beds  of  Wyoming  and  the  Purbeck  of  England,  three 
genera  are  known  from  the  Jura,  all  found  in  European  localities  : 
Phascolotherium,  Amphitherium,  and  Stereognathus,  all  of  them 
tiny  and  very  primitive  creatures.  From  the  scanty  remains  it  is 
not  possible  to  learn  much  about  them. 


CHAPTER   XXX 
THE  CRETACEOUS  PERIOD 

THE  name  Cretaceous  is  derived  from  the  Latin  word  for  chalk 
(Creta),  because  in  England,  where  the  name  was  early  used,  the 
thick  masses  of  chalk  are  the  most  conspicuous  members  of  the 
system. 

In  very  marked  contrast  to  the  scanty  development  of  the  Jura, 
the  Cretaceous  strata  of  North  America  are  displayed  on  a  vast 
scale,  and  cover  enormous  areas  of  the  continent,  eloquent  wit- 
nesses of  the  great  geographical  changes  in  that  long  period. 
Fresh-water,  estuarine,  and  marine  rocks  are  all  well  represented, 
and,  in  consequence,  our  information  regarding  the  life  of  North 
America  and  its  seas  during  Cretaceous  times  is  incomparably 
more  complete  than  it  is  for  the  Triassic  and  Jurassic. 

The  Cretaceous  rocks  of  North  America  are  of  very  different 
character  in  the  different  parts  of  the  continent,  and  require  sepa- 
rate classification. 

DISTRIBUTION  OF  CRETACEOUS  ROCKS 

American. — At  the  opening  of  the  Cretaceous,  the  Atlantic 
coast  of  North  America  appears  to  have  been  farther  to  the  east- 
ward than  it  is  at  present ;  but  just  as  had  happened  in  the  Triassic 
period,  a  long,  narrow  depression  was  formed,  running  roughly 
parallel  with  the  coast,  and  in  this  depression  one  or  more  bodies 
of  water  accumulated,  in  which,  for  a  long  period  of  time,  sedi- 
ments in  the  form  of  sands  and  clays  were  deposited.  This 
Potomac  series,  which  is  divisible  into  several  stages,  has  been 
traced  through  the  islands  of  Martha's  Vineyard,  Nantucket, 
Block  Island,  Long  Island,  Staten  Island,  across  New  Jersey,  and 

474 


CRETACEOUS  OF  THE  UNITED  STATES 


475 


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Livingstone  or 
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Laramie  Stage. 

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Colorado  Stage 
(Belly  River). 
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i.  Benton. 

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4/6  THE   CRETACEOUS    PERIOD 

thence  southward  to  Georgia,  where  it  turns  northwestward,  fol- 
lowing the  Mississippi  embayment  into  Tennessee,  and  from  there 
turning  southwestward  through  Arkansas.  In  the  northern  part 
of  this  region,  from  Nantucket  to  the  Delaware  River,  only  the 
uppermost  part  of  the  Potomac  series  has  been  found.  The 
Potomac  is  nowhere  marine,  and  everywhere  rests  unconformably 
upon  the  underlying  Triassic  and  older  rocks.  The  accumulation 
of  sediments  in  these  depressions  went  on  for  a  very  long  time, 
apparently  throughout  the  whole  of  the  Lower  Cretaceous,  and  by 
some  geologists  it  is  believed  that  the  process  began  in  the  Juras- 
sic, to  which  period  they  refer  the  lowest  members  of  the  Potomac 
series.  As  the  thickness  of  sediment  is  not  great  (not  exceeding 
600  feet),  the  process  of  deposition  must  have  been  very  slow. 

While  along  the  Atlantic  border  the  land  was  more  extended 
than  at  present,  in  the  southern  part  of  the  continent  a  different 
order  of  events  was  brought  about.  In  southern  Mexico  occurred 
a  depression  which  submerged  the  land  almost  from  ocean  to 
ocean,  through  probably  leaving  some  sort  of  land  barrier  between 
the  Atlantic  and  the  Pacific.  The  transgression  of  the  sea  ex- 
tended northward,  covering  most  of  Texas  and  Oklahoma,  and 
sending  a  bay  into  southern  Kansas.  At  the  base  of  the  Lower 
Cretaceous  strata  in  Texas  is  found  a  deposit  of  fresh-water  sands, 
the  Trinity  stage,  which  is  the  recognized  equivalent  of  the  basal 
Potomac.  The  advancing  sea  soon  obliterated  this  body  of  fresh 
water,  and  the  continued  depression  soon  established  a  clear  and 
quite  deep  sea,  in  which  were  formed  the  great  masses  of  the 
Comanche  limestones,  that  are  the  surface  rocks  of  nearly  all 
Mexico  and  cover  a  large  part  of  Texas.  The  Ouachita  Moun- 
tains of  Arkansas  stood  out  as  a  promontory  in  the  Lower  Creta- 
ceous sea  and  the  ancient  shore  line  has  been  traced  around  their 
foot.  Over  much  of  Texas  the  Comanche  limestones  are  soft, 
and  beds  of  chalk  occur  among  them  ;  while  in  Mexico,  where  they 
have  been  folded  into  mountain  ranges,  they  have  become  much 
harder  and  more  compact.  The  thickness  of  the  limestones  in- 
creases southward ;  from  1000  feet  in  northern  central  Texas,  it 
rises  to  5000  feet  on  the  Rio  Grande,  and  on  the  Mexican  plateau 


AMERICAN  477 

the  almost  incredible  thickness  of  10,000  to  20,000  feet  has  been 
reported.  No  less  than  six  distinct,  successive  marine  faunas  are 
found  in  the  Comanche  limestones  of  Texas,  and  the  faunal  rela- 
tionships of  this  region  are  closest  with  the  Mediterranean  province 
of  Europe,  and  especially  with  the  Lower  Cretaceous  of  Portugal. 

In  the  northern  interior  region  the  Lower  Cretaceous  beds  were 
all  laid  down  in  inland  bodies  of  water,  part  of  which,  at  least, 
were  fresh.  One  such  body  of  water  covered  southern  Wyoming, 
extending  down  the  eastern  flank  of  the  Rocky  Mountains  into 
Colorado.  In  it  were  deposited  a  thin  mass  of  sands  and  clays, 
the  Como  Beds?  in  which  are  preserved  the  remains  of  a  rich  land 
fauna  of  reptiles  and  mammals.  These  beds  are  usually  referred 
to  the  summit  of  the  Jurassic,  but  their  near  equivalence  seems  to 
be  with  the  Trinity  of  Texas,  the  basal  Potomac  of  the  Atlantic 
border,  and  with  the  English  Wealden,  all  of  which  are  well-nigh 
universally  regarded  as  Cretaceous.  Another  non-marine  and,  at 
least  locally,  fresh  body  of  water  occurred  east  of  the  Gold  Range 
of  British  Columbia,  extending  southward  into  Montana,  and  in  it 
were  deposited  the  sands  and  clays  of  the  Kootanie  stage,  the 
plant  remains  of  which  correlate  it  with  the  lower  Potomac,  though 
it  may  have  been  considerably  later  than  the  Como  beds.  In  this 
northern  area  there  is  no  evidence  of  deep  water,  but  only  of 
shallow  seas  or  lakes,  with  tracts  of  low-lying,  swampy  lands,  on 
which  a  luxuriant  vegetation  produced  valuable  deposits  of  coal. 
Other  inland  waters  occupied  an  unknown  extent  of  the  Great 
Plains  area ;  Lower  Cretaceous  beds  have  been  found  surrounding 
the  Black  Hills  and  in  a  few  other  localities. 

Along  the  Pacific  coast  Lower  Cretaceous  rocks  are  displayed 
on  a  great  scale.  The  Great  Basin  land  then  extended  from 
southern  Nevada  to  latitude  54°  N.  in  British  Columbia,  with  the 
Sierra  Nevada  rising  along  part  of  its  western  shore,  to  which  the 
Pacific  extended.  North  of  the  Gold  Range  in  British  Columbia, 
the  ocean  spread  eastward,  though  no  doubt  broken  by  many 

1  These  beds  were  named  by  their  describer,  Professor  Marsh,  the  Atlantosaurus 
Beds,  but  as  this  name  ^'inadmissible,  the  term  Como  Stage  may  be  substituted, 
from  Como,  Wyoming,  the  typical  locality. 


4/8  THE   CRETACEOUS   PERIOD 

islands,  to  the  eastern  base  of  the  Rocky  Mountains.  The  Coast 
Range  of  California  formed  a  chain  of  islands  and  reefs.  In  the 
Sierra  Nevada  occurs  an  unconformity  between  the  Lower  Creta- 
ceous and  the  uppermost  Jurassic,  but  it  does  not  imply  the  lapse 
of  a  very  long  period  of  time. 

The  older  division  of  the  Californian  Lower  Cretaceous  is  called 
the  Knoxville,  and  has  an  estimated  maximum  thickness  of 
20,000  feet,  laid  down  upon  a  slowly  subsiding  sea-bottom.  At 
the  end  of  the  Knoxville  age,  the  subsidence  became  more  rapid 
and  the  sea  began  to  encroach  upon  the  land,  for  the  Horsetown 
beds,  which  have  a  thickness  of  6000  feet,  overlap  the  Knoxville 
shoreward  and  extend  over  upon  the  underlying  Jurassic  and  other 
pre-Cretaceous  systems.  Although  the  two  stages  of  the  Califor- 
nian Lower  Cretaceous  are  entirely  conformable  throughout,  and 
appear  to  have  been  formed  by  a  continuous  process  of  sedimen- 
tation, yet  there  is  a  very  marked  faunal  change  between  them. 
The  Knoxville  beds  have  a  northern  fauna,  allied  to  that  of  Russia, 
showing  that  the  connection  with  Russian  seas,  which  had  been 
established  in  late  Jurassic  times,  was  still  kept  up.  With  the 
beginning  of  the  Horsetown  age,  however,  this  northern  commu- 
nication was  interrupted,  and  a  connection  was' formed  with  the 
waters  of  southern  Asia,  and  in  that  way  with  central  Europe. 
The  decided  contrast  which  we  find  between  the  Lower  Cretaceous 
faunas  of  California  and  of  Texas  points  to  the  existence  of  a  land 
barrier  between  the  seas  of  the  two  regions. 

In  the  southern  region  the  Lower  Cretaceous  was  terminated 
by  a  great  upheaval,  which  over  most  of  Mexico  and  Texas  caused 
the  ocean  to  retire  nearly  to  its  present  position,  raising  at  the 
same  time  a  long  ridge  of  land  which  became  connected  with  the 
Great  Basin  land.  This  mid-Cretaceous  land  epoch  must  have 
continued  for  a  considerable  time,  permitting  extensive  denuda- 
tion and  a  complete  change  in  the  fauna.  Wherever  the  Upper 
Cretaceous  is  in  contact  with  the  Comanche  limestones,  the  two 
are  unconformable,  and  no  species  of  animal  is  known  to  pass 
from  one  to  the  other.  These  limestones  form  the  principal  mass 
of  the  Mexican  mountains,  where  the  force  of  compression  has 


AMERICAN  479 

converted  them  into  rocks  of  great  density,  and  from  their  ancient 
appearance  they  were  long  supposed  to  be  Carboniferous. 

The  Upper  Cretaceous  rocks  have  a  far  wider  distribution  over 
North  America  than  have  those  of  the  lower  division,  which  is  due 
to  an  enormous  transgression  of  the  sea  over  the  land,  one  of  the 
greatest  in  all  recorded  geological  history.  Over  the  region  of 
the  Great  Plains  the  Upper  Cretaceous  was  inaugurated  by  the 
formation  of  a  non-marine  stage,  the  Dakota.  These  strata  cover 
much  of  Texas,  lying  unconformably  upon  the  Comanche  series, 
and  extend  northward  into  Canada.  It  is  very  difficult  to  com- 
prehend under  what  conditions  these  vast  sheets  of  conglomerate 
and  sandstone  could  have  been  laid  down,  and  there  is  much 
reason  to  believe  that  not  all  the  beds  referred  to  the  Dakota  really 
belong  to  it.  On  the  western  side  of  the  Colorado  uplift,  the 
Dakota  is  less  distinctly  a  sandstone  formation,  and  is  charac- 
terized by  beds  of  shale  and  even  coal  seams  of  workable  thick- 
ness. In  most  parts  of  the  Rocky  Mountain  region  the  Dakota 
rests  in  apparent  conformity  upon  the  lowest  fresh-water  Creta- 
ceous, and  even  upon  the  Jurassic.  In  the  Uinta  and  Wasatch 
ranges  there  is  no  apparent  break  in  sedimentation  from  the 
Palaeozoic  to  the  end  of  the  Cretaceous,  though  the  whole  Lower 
Cretaceous  is  there  wanting.  From  this  we  may  infer  that  during 
the  long  Lower  Cretaceous  time  all  these  regions  had  been  low- 
lying  lands,  nearly  or  quite  at  base-level,  and  therefore  not  subject 
to  profound  denudation. 

It  was  at  the  end  of  the  Dakota  age  that  the  great  subsidence 
took  place  which  affected  nearly  all  parts  of  the  continent,  and 
brought  the  sea  in  over  vast  areas  where  for  ages  had  been  dry 
land.  South  of  New  England  the  Atlantic  coastal  plain  was  sub- 
merged, and  in  New  Jersey,  at  least,  the  waters  covered  even  the 
Triassic  belt,  bringing  the  sea  up  to  the  foot  of  the  crystalline 
highlands.  The  lowlands  of  Maryland,  Virginia,  and  the  Caro- 
linas,  and  all  of  Florida  were  under  the  ocean,  and  the  Gulf  of 
Mexico  was  extended  northward  in  a  great  bay  (the  Mississippi 
embayment),  covering  western  Tennessee  and  Kentucky  and  ex- 
tending into  southern  Illinois.  Northeastern  Mexico  and  Texas 


480  THE  CRETACEOUS   PERIOD 

were  again  submerged,  and  a  wide  sea  connected  the  Gulf  of 
Mexico  with  the  Arctic  Ocean.  The  eastern  coast  of  this  interior 
sea  began  in  northwestern  Texas,  running  through  Kansas  and 
Iowa  nearly  to  the  present  line  of  the  Mississippi  River.  West- 
ward the  coast-line  was  the  uplift  which  ran  from  southern  Mexico 
into  British  Columbia.  The  Colorado  region  was  again  converted 
into  islands.  North  of  the  Great  Basin  land  the  interior  sea  was 
connected  with  the  Pacific  and  Arctic  Oceans,  which  united  over 
the  northwestern  part  of  the  continent. 

On  the  Pacific  side,  the  Sierras,  which  had  suffered  greatly  from 
denudation,  were  again  folded,  and  separated  from  the  interior 
basin  by  a  fault,  while  a  fracturing  of  the  crust  began  the  system  of 
Basin  Ranges,  arching  upward  the  surface  of  the  Great  Basin.  A 
moderate  transgression  of  the  sea  caused  the  Upper  Cretaceous  to 
extend  farther  east  than  the  Lower.  Volcanic  activity  continued 
and  immense  bathyliths  were  formed  deep  within  the  mountains. 
The  sea  extended  from  Lower  California  northward  along  the  Sierra, 
into  eastern  Oregon  at  the  foot  of  the  Blue  Mountains. 

The  North  American  continent  was  thus  divided  into  two  prin- 
cipal land  masses,  the  larger  one  to  the  east  and  comprising  the 
pre-Cambrian  and  Palaeozoic  areas.  In  the  limits  of  the  United 
States  this  land  lay  almost  entirely  east  of  the  Mississippi,  except 
for  a  southwestern  peninsula,  including  Missouri,  Arkansas,  Okla- 
homa, and  part  of  Texas.  The  western  area  was  much  smaller, 
extending  from  southern  Mexico  into  British  Columbia,  and  hav- 
ing its  greatest  width  between  the  fortieth  and  forty-fifth  parallels 
of  latitude.  Between  the  two  lands  lay  the  Colorado  islands,  and 
doubtless  many  smaller  ones  as  well. 

The  character  of  sedimentation  differed  so  much  in  the  various 
regions  of  the  continent  that  the  subdivisions  of  the  Upper  Creta- 
ceous have  received  different  names  in  the  separate  provinces,  and 
only  approximately  correspond  in  time. 

Along  the  Atlantic  border  the  Upper  Cretaceous  strata  are  a 
series  of  marine  sands  and  clays,  which  are  still  almost  horizontal 
in  position  and  of  loose,  incoherent  texture.  In  New  Jersey  there 
are  extensive  developments  of  green  sands  (see  p.  175),  locally 


AMERICAN  481 

called  marl.  The  Appalachian  Mountains,  which  had  been  sub- 
jected to  the  long-continued  denudation  of  Triassic,  Jurassic,  and 
Lower  Cretaceous  times,  were  now  reduced  nearly  to  base-level, 
the  Kittatinny  plain  of  geographers  (see  p.  342).  This  peneplain 
was  low  and  flat,  covering  the  whole  Appalachian  region,  and  the 
only  high  hills  upon  it  were  the  mountains  of  western  North  Caro- 
lina, then  much  lower  than  now.  Across  this  low  plain  the  Dela- 
ware, Susquehanna,  and  Potomac  must  have  held  very  much  their 
present  courses,  meandering  through  alluvial  flats. 

On  the  Gulf  border  the  Upper  Cretaceous  beds  of  Alabama  and 
Mississippi  are  limestones  called  the  Rotten  Limestone  below  (500 
to  1200  feet  thick),  and  the  Ripley  above  (200  feet).  Eastward 
the  water  shallowed,  and  in  Georgia  we  find  about  1400  feet  of 
clays  and  sands.  Northward  along  the  Mississippi  embayment  the 
beds  thin  greatly  and  are  mostly  clays  and  sands. 

In  the  interior  region  lying  upon  the  Dakota  are  the  marine 
beds  of  the  Colorado,  of  which  the  lower  division  is  the  Benton, 
a  mass  of  shales  and  limestones  with  a  maximum  thickness  of 
1000  feet,  though  varying  much  from  point  to  point.  The  depres- 
sion still  continuing,  the  sea  became  quite  deep,  making  favourable 
conditions  for  the  formation  of  the  chalk  and  harder  limestones 
of  the  Niobrara.  This  chalk  is  best  seen  in  Kansas,  but  extends 
into  South  Dakota ;  elsewhere  are  sandstones  and  limestones  with 
a  maximum  thickness  of  2000  feet.  A  movement  of  reelevation 
of  the  sea-bottom  began  even  in  Colorado  times,  and  in  the 
northern  part  of  the  interior  region  oscillations  of  level  produced 
alternating  fresh-water,  or  estuarine,  and  marine  conditions.  In 
Montana  and  the  Canadian  province  of  Alberta  is  a  thick  body 
of  estuarine  or  fresh- water  strata  with  seams  of  coal  (the  Belly 
River  formation)  interposed  between  the  marine  deposits  of  the 
Colorado  below  and  the  Montana  above.  In  Utah  is  another 
fresh-water  deposit  of  coal-bearing  rocks  of  Colorado  age. 

In  the  Montana  stage  marine  conditions  still  prevailed,  but  the 
waters  of  the  northern  sea  had  generally  become  much  shallower, 
and  a  marked  change  of  fauna  is  produced.  In  Alberta  are  coal 
measures  of  this  date.  Two  divisions  of  the  Montana  are  distin- 


482  THE  CRETACEOUS   PERIOD 

guished,  although  not  everywhere  separable ;  the  Fort  Pierre, 
which  is  composed  of  shales  and  sandstones  with  a  maximum 
thickness  of  8000  feet,  and  the  Fox  Hills,  sandstones  and  some 
shales,  which  do  not  exceed  1000  feet.  This  movement  of  up- 
heaval in  the  interior  was  accompanied  or  followed  by  an  uplift 
on  the  Atlantic  and  Gulf  coasts,  for  along  these  borders  the  upper- 
most Cretaceous  beds  are  either  wanting  or  represented  by  exceed- 
ingly thin  deposits.  In  the  interior  the  continued  upheaval  caused 
estuarine,  fresh-water,  and  swampy  conditions  to  prevail  over  very 
wide  areas,  though  not  so  widely  extended  as  had  been  the  Upper 
Cretaceous  sea.  The  older  part  of  this  great  fresh-  and  brackish- 
water  formation  is  the  Laramie.  The  northwestern  part  of  the 
continent  had  been  converted  into  dry  land,  but  a  broad  estuary 
extended  up  the  course  of  the  present  Mackenzie  River  to  lati- 
tude 62°  N.  Another  and  vastly  larger  body  of  water  began 
about  latitude  57°  N.  and  reached,  though  perhaps  with  interrup- 
tions, to  northeastern  Mexico,  surrounding  the  Colorado  island. 
This  great  inland  sea  was  2000  miles  long  and  500  miles  wide, 
though  it  is  by  no  means  certain  that  all  of  it  was  under  water  at 
the  same  time.  In  the  swamps  and  shallows  were  gathered  great 
quantities  of  vegetable  matter,  now  converted  into  coal  seams. 
Workable  coal  is  found  in  all  the  stages  of  the  western  Cretaceous, 
but  none  of  these  stages  is  comparable  to  the  Laramie  for  the 
extent  and  thickness  of  its  coal  measures.  In  the  Laramie  sea 
were  alternating  conditions  of  fresh  and  brackish  water  and,  it  is 
said,  occasional  inroads  from  the  ocean  occurred. 

The  Laramie  was  a  time  of  tranquillity,  with  only  slow  and  gentle 
changes  of  level,  but  towards  its  close  .some  important  disturb- 
ances took  place,  especially  along  the  Rocky  Mountains.  The 
first  of  these  movements  affected  only  the  Colorado  island,  and 
its  effects  are  especially  well  shown  in  the  Denver  basin,  where 
some  800  feet  of  conglomerates  (the  Ampahoe)  rest  upon  the 
Laramie  unconformably.  The  second  series  of  movements  was 
much  more  extensively  felt,  producing  marked  unconformities  both 
in  Colorado  and  Montana.  In  Colorado  there  was  a  great  vol- 
canic outburst,  and  the  Denver  stage,  which  overlies  the  Arapahoe 


FOREIGN  483 

unconformably,  is  principally  composed  of  andesitic  tuffs.  In 
Montana  the  equivalent  stage  {Livingstone)  is  7000  feet  thick 
and  unconformable  with  the  Laramie. 

The  Upper  Cretaceous  of  the  Pacific  coast  comprises  the  Chico 
series,  with  a  maximum  thickness  of  4000  feet.  In  Vancouver's 
Island  the  Chico  is  coal-bearing.  The  faunal  connections  of  the 
Chico  are  with  southern  Asia,  that  series  having  very  little  in  com- 
mon with  those  of  the  interior  region.  The  uppermost  Cretaceous 
is  wanting  along  the  Pacific  coast,  except  for  certain  coal-bearing 
beds  in  Washington,  which  appear  to  represent  the  Laramie. 

The  Mesozoic  era  was  closed  in  the  West,  as  the  Palaeozoic  had 
been  in  the  East,  by  a  time  of  great  mountain  making,  and  to  this 
movement  is  attributed  the  formation  of  most  of  the  great  Western 
mountain  chains.  From  the  Arctic  Ocean  far  into  Mexico  the 
effects  of  the  disturbance  were  apparent.  The  Rocky  Mountains, 
the  Wasatch  and  Uinta  ranges,  the  high  plateaus  of  Utah  and 
Arizona,  and  the  mountains  of  western  Texas  and  Mexico  date 
from  this  time,  though  subsequent  movements  have  greatly  modi- 
fied them.  Vast  volcanic  disturbances  accompanied  the  upheaval, 
which  was  on  a  far  grander  scale  than  the  Appalachian  revolution. 

Foreign.  —  In  South  America  the  Cretaceous  history  is  much 
like  that  of  the  northern  continent.  The  subsidence  which  inau- 
gurated the  Lower  Cretaceous  extended  the  sea  over  the  north- 
ern part  of  South  America  and  covered  northeastern  Brazil,  with 
fresh-water  deposits  in  central  Brazil.  All  along  the  Cordillera, 
from  Venezuela  to  Patagonia,  marine  Cretaceous  is  found,  but  east 
of  the  mountains,  with  the  exceptions  already  noted,  the  system  is 
represented  chiefly  by  non-marine  sandstones.  The  faunal  rela- 
tions of  the  South  American  Lower  Cretaceous  are  very  intimate 
with  northern  and  western  Africa.  Gigantic  volcanic  activity  went 
on  along  the  Cordillera  in  Mesozoic  times ;  in  Chili  and  Peru  the 
marine  Cretaceous  is  principally  made  up  of  stratified  igneous  ma- 
terial, and  the  Andes  contain  the  largest  known  area  of  Mesozoic 
eruptives.  The  mountain-making  upheaval  probably  came  at  the 
close  of  the  Cretaceous. 

In  Europe,  toward  the  end  of  the  Jura,  the  sea  retired  from 


484  THE   CRETACEOUS    PERIOD 

nearly  all  of  the  central  region,  which  in  part  became  dry  land 
and  in  part  was  covered  with  lakes  and  inland  seas.  One  of  the 
largest  of  these  covered  much  of  southern  England,  extending  far 
into  Germany,  and  in  it  was  deposited  a  great  thickness  of  sand 
and  clay,  with  some  shell  limestone,  the  Wealden.  The  Alpine  re- 
gion remained  submerged  under  a  clear  and  deep  sea,  and  the  tran- 
sition from  the  Jurassic  is  very  gradual.  In  the  oldest  Cretaceous 
epoch  {Neocomiari}  a  renewed  transgression  submerged  large 
parts  of  central  Europe,  though  the  sea  was  far  less  extensive  than 
that  of  the  Middle  and  Upper  Jurassic.  In  consequence,  a  great 
gulf  was  established  over  southern  England,  northern  France,  and 
north  Germany  to  Poland,  a  gulf  bounded  on  the  north  by  the 
highlands  of  Britain,  Scandinavia,  and  northwestern  Russia,  and  on 
the  south  by  a  land  stretching  from  Ireland  to  Bohemia ;  Belgium 
was  an  island.  The  expanded  Mediterranean  covered  south- 
eastern Asia  Minor  and  northern  Africa.  In  the  Upper  Cretaceous 
the  northern  gulf  was  greatly  extended,  covering  many  areas  that 
had  been  land  since  Palaeozoic  or  pre-Cambrian  times.  Parts 
of  this  basin  became  very  deep,  and  its  most  characteristic  de- 
posit, especially  over  southern  England  and  northern  France,  was 
chalk,  which  the  microscope  shows  to  be  made  up  of  the  shells  of 
Foraminifera  and  to  greatly  resemble  the  modern  foraminiferal 
oozes  (see  p.  215).  Over  the  Alpine  region  upheavals  in  the 
Upper  Cretaceous  had  established  land  areas,  indicated  by  exten- 
sive fresh-water  deposits,  recurring  at  intervals  from  Spain  to 
Hungary,  in  the  latter  country  containing  coal.  The  Cretaceous 
was  closed  in  Europe  by  a  gradual  upheaval  which  excluded  the 
sea  from  wide  areas  that  it  had  occupied. 

In  Africa  the  only  extensive  Cretaceous  areas  are  those  of  the 
north,  where  the  Atlas  Mountains  and  much  of  the  surface  of 
the  Libyan  desert  are  made  up  of  these  rocks.  A  limited  trans- 
gression of  the  sea  also  took  place  along  the  western  coast  and 
another  on  the  east  coast  of  Cape  Colony  and  Natal. 

Southern  and  eastern  Asia  display  many  areas  of  Cretaceous 
rocks,  as,  for  example,  in  southern  India  and  Japan.  Australia 
also  has  extensive  areas  of  this  system,  which  are  best  known  in 


PLANTS 


485 


Queensland,  where  they  are  chiefly  Lower  Cretaceous  and  contain 
coal.     The  New  Zealand  Cretaceous  is  also  coal-bearing. 


CRETACEOUS  LIFE 

The  life  of  the  Cretaceous  displays  so  great  an  advance  over  that 
of  the  Jurassic  that  the  change  may  fairly  be  called  a  revolution. 

Plants.  —  If  the  separation  between  the  Mesozoic  and  Cenozoic 
eras  were  made  entirely  with  reference  to  the  plants,  it  would  pass 


FIG.  159.  —  Cretaceous  leaves,  Dakota  stage,    i.  Dammarites  emarginatus,  1/2. 
2.  Betulites  Westi,  3/4.    3.  Liriodendron  giganteum,  1/2.     (After  Lesquereux.) 

between  the  Jurassic  and  the  Cretaceous,  just  as  a  similar  criterion 
would  remove  the  Upper  Permian  to  the  Meozoic  (see  p.  432). 
The  vegetation  of  the  Lower  Cretaceous,  especially  of  the  lowest, 
is  still  much  like  that  of  the  Jura.  Ferns,  Horsetails,  Cycads,  and 
Conifers  continue  to  make  up  most  of  the  flora,  but  the  impend- 
ing revolution  is  announced  by  the  appearance  of  Dicotyledons  of 
archaic  and  primitive  type.  In  the  higher  parts  of  the  Potomac 
the  Cycads  become  much  less  abundant  and  the  Dicotyledons  very 
much  more  so.  Here  we  find  many  leaves  which  belong  to  genera 
that  cannot  be  distinguished  from  those  of  modern  forest  trees, 
such  as  Sassafras,  Populus,  Liriodendron,  etc.  No  Dicotyledons 


486  THE  CRETACEOUS   PERIOD 

have  been  found  in  the  Kootanie  of  the  Northwest,  or  in  the 
Wealden  of  northern  Europe,  but  they  occur  in  the  Lower  Cre- 
taceous of  Portugal.  In  the  latter  part  of  the  Lower  and  in  all 


FIG.  160.  —  Sassafras  dissectum,  1/2.     Dakota  stage.     (Lesquereux.) 

the  Upper  Cretaceous,  the  flora  assumes  an  almost  completely 
modern  character,  and  nearly  all  of  our  common  kinds  of  forest 
trees  are  represented  :  Sassafras,  Poplars,  Willows,  Oaks,  Maples, 
Elms,  Beeches,  Chestnuts,  and  very  many  others,  A  new  element 
is  the  Monocotyledonous  group  of  Palms,  which  speedily  assume 


ECHINODERMATA  487 

great  importance.  Each  successive  plant-bearing  horizon  of  the 
Cretaceous  is  characterized  by  its  own  special  assemblage  of  plants, 
but  in  its  general  features  the  Upper  Cretaceous  flora  is  essentially 
modern.  Cretaceous  animals  are  sufficiently  different  from  those 
of  the  Jura,  but  the  change  is  not  so  revolutionary  as  we  have 
found  among  the  plants. 


FIG.  161.  —  Cinnamomum  affine,  1/2.     Laramie  stage.     (Lesquereux.) 

Foraminifera  play  an  important  part  in  the  construction  of  Creta- 
ceous rocks,  especially  of  the  great  masses  of  chalk,  while  the  green 
sands  are  casts  of  foraminiferal  shells  in  glauconite.  The  most 
abundant  genus,  as  in  the  recent  Atlantic  oozes,  is  Globigerina. 

Spongida.  —  In  the  Cretaceous  of  Europe  Sponges  are  more  nu- 
merous and  varied  than  at  any  other  time,  but  in  North  America 
they  are  far  less  common. 

Ccelenterata.  — The  Corals  were  very  much  as  they  are  to-day 
and  require  no  special  description. 

The  Echinodermata  undergo  some  very  marked  changes.  The 
Crinoids  are  much  reduced  since  the  Jurassic,  and  never  again 
assume  their  ancient  importance  ;  characteristic  Cretaceous  genera, 
are  the  steiriless  Uintacrinus  (PL  X,  Fig.  i)  and  Marsupites.  The 
Sea-urchins  are  incomparably  more  numerous  in  Europe  than  in 
North  America ;  of  the  Regular  forms  may  be  mentioned  Pseudo- 
diadema  (X,  2),  CidaHs,  and  Salema,  and  of  the  Irregular  forms, 
Toxaster  (X,  3),  Holaster,  Cassidulus,  etc. 


488  THE  CRETACEOUS   PERIOD 

Arthropoda.  —  Among  the  Crustacea  we  need  only  note  the 
great  increase  in  the  Brachyuran  Decapods,  or  Crabs. 

Brachiopoda  are  very  much  as  in  the  Jurassic  ;  the  common 
genera  are  Terebratula  (X,  4),  Terebratella  (X,  5),  and  Rhyncho- 
nella. 

Mollusca.  —  This  group  is  very  richly  developed  and  many 
genera  are  peculiar  to  the  period.  The  large,  curious  oysters  be- 
longing to  the  genera  Ostrea  (X,  6),  Gryphcea,  and  especially 
Exogyra,  are  common,  and  the  many  species  of  Inoceramus  (X,  7) 
are  very  characteristic.  Confined  to  the  Cretaceous  are  the  ex- 
traordinary shells  classed  as  Rudistes,  in  which  one  valve  is  long 
and  horn-shaped,  and  the  other  a  mere  cover  for  it.  These  shells 
of  the  genera  Hippurites,  Radiolites •,  and  Coralliochama  are  much 
commoner  in  Europe  than  in  America.  Other  peculiar  Cretaceous 
Bivalves  are  Requienia,  Caprotina  (X,  8),  and  Auce.Ua  (X,  18), 
the  latter  also  Jurassic.  The  Gastropods  (PI.  X,  Figs.  9,  10,  n), 
are  very  much  as  in  the  Jura,  but  in  the  latter  part  of  the  period 
come  in  many  genera  which  reach  their  fullest  development  in 
Tertiary  and  recent  times,  such  as  Fusus,  Murex,  Valuta,  Cyprcea, 
and  many  others. 

The  Cephalopods  are  very  peculiar ;  in  addition  to  numerous 
Ammonoid  genera  with  closely  coiled  shells  of  normal  type,  such 
as  Hoplites,  Schl&nbachia,  Placenticeras,  we  find  very  many 
shells  entirely  or  partially  uncoiled,  or  rolled  up  in  peculiar  ways, 
which  give  to  the  Cretaceous  Cephalopod  fauna  a  character  all  its 
own.  In  Crioceras  the  shell  is  coiled  in  an  open,  flat  spiral,  the 
whorls  of  which  are  not  in  contact.  Ancyloceras  has  a  similar 
open  coil,  followed  by  a  long,  straight  portion,  and  recurved  ter- 

EXPLANATION  OF  PLATE  X,  p.  489.  i.  Uintacrinus  socialis,  1/12.  (Clark). 
2.  Pseudodiadema  texanum.  (Clark.)  3.  Toxaster  texanus.  (Conrad.)  4.  Tere- 
bratula Harlani,  3/4.  (Whitfield.)  5.  Terebratella  plicata.  (Whitfield.)  6.  Ostrea 
larva.  (Whitfield.)  7.  Inoceramus  problematicus,  3/4.  (Meek.)  8.  Caprotina 
bicornis,  1/3.  (Meek.)  9.  Fasciolaria  buccinoides.  (Meek.)  10.  Anchura  ameri- 
cana.  (Meek.)  n.  Margarita  nebrascensis.  (Meek.)  12.  Ptychoceras  Mortoni, 
3/4.  (Meek.)  13.  Scaphites  nodosus,  1/2.  14.  Baculites  compressus,  1/2. 
(Meek.)  15.  Belemnitella  americana,  1/2.  (Whitfield.)  16.  Nodosaria  texana, 
enlarged.  (Conrad.)  17.  Micrabacia  americana,  3/1.  (Meek.)  18.  Aucella 
Piochi.  (Gabb.) 


CRETACEOUS   FOSSILS 


489 


\ 


PLATE  X.   AMERICAN  CRETACEOUS  FOSSILS 


490  THE  CRETACEOUS   PERIOD 

minal  chamber.  Scaphites  (X,  13)  is  like  a  shortened  Ancyloceras. 
In  Ptychoceras  (X,  12)  the  shell  consists  of  two  parallel  parts, 
connected  by  a  single  sharp  bend.  Turrilites  is  coiled  into  a  high 
spiral,  like  a  Gastropod,  and  Baculites  (X,  14)  has  a  perfectly 
straight  shell  except  for  a  minute  coil  at  one  end.  Nautilus  is 
represented  by  many  species,  some  of  them  very  large.  Belem- 
nites  are  very  abundant,  but  in  the  Upper  Cretaceous  the  genus 
Belemnitella  (X,  15)  replaces  the  true  Belemnites. 

The  Vertebrata  form  the  most  characteristic  element  of  the 
Cretaceous  fauna.  Among  the  Fishes  a  revolution  has  occurred. 
Sharks  of  modern  type  abound,  and  their  teeth  are  found  in  count- 
less numbers ;  but  the  principal  change  consists  in  the  immense 
expansion  of  the  Teleosts  or  Bony  Fishes,  which  now  take  the 
dominant  place,  while  Ganoids  become  rare.  Most  of  the  Creta- 
ceous Teleosts  belong  to  modern  families  and  even  genera,  such 
as  the  Herrings,  Cod,  Salmon,  Mullets,  Catfishes,  etc. ;  but  a 
characteristic  Cretaceous  type,  now  extinct,  is  that  of  the  Sauro- 
donts,  fierce,  carnivorous  fishes  of  great  size  and  power.  The 
genus  Portheusy  common  in  the  Kansas  chalk,  was  12  to  15  feet 
long,  and  was  provided  with  great,  reptile-like  teeth. 

The  Reptiles  continued  to  be  the  dominant  types  of  the  land, 
the  sea,  and  the  air,  and  it  may  fairly  be  questioned  whether  the 


FIG.  162.  —  Clidastes  velox,  1/96.     (Williston.) 

Jura  or  the  Cretaceous  should  be  regarded  as  the  culminating 
period  of  Reptilian  history.  Ichthyosaurs  and  Plesiosaurs  are 
perhaps  less  abundant  than  in  the  Jura,  but  are  of  greatly  in- 
creased size.  Elasmosaurus,  a  Plesiosaur  from  the  Kansas  chalk, 
had  a  length  of  40  to  50  feet,  of  which  22  feet  belonged  to  the 
slender  neck.  Confined  to  the  Cretaceous  are  the  remarkable 
marine  reptiles  of  the  group  Pythonomorpha,  or  Mosasauria,  which 
swarmed  on  the  Atlantic  and  Gulf  coasts,  and  especially  in  the 


DINOSAURIA  491 

interior  sea.  These  were  gigantic,  carnivorous  marine  lizards, 
with  the  limbs  converted  into  swimming  paddles  (see  Fig.  162). 
Turtles,  both  fresh-water  and  marine,  abound,  and  some  were  very 
large.  Lizards  and  Snakes  are  but  scantily  represented,  not  dis- 
playing the  manifold  variety  of  structure  which  they  afterwards 
acquired.  Crocodiles,  like  those  of  modern  days,  were  ubiquitous 
in  both  fresh  and  salt  waters. 

The  Pterosaurs  of  the  Cretaceous  are  remarkable  for  their 
great  size,  far  exceeding  that  of  the  Jurassic  species.  Ornitho- 
stoma,  which  has  been  found  both  in  Kansas  and  in  Europe,  had  a 
head  of  nearly  3  feet  in  length,  with  a  long,  pointed,  toothless 
bill,  like  that  of  a  bird ;  the  spread  of  wings  exceeded  20  feet. 

The  Dinosaurs  continue  in  even  greater  profusion  than  in  the 
Jurassic ;  they  are,  of  course,  much  commoner  and  better  pre- 
served in  fresh-water  deposits  than  in  marine,  and  hence  are  best 
known  from  the  base  and  the  summit  of  the  system.  Many  of  the 
genera  were  the  largest  land  animals  that  ever  lived,  and  the  size 
of  the  bones  is  astonishing.  Ornithopsis,  Diplodocus,  and  Cetio- 
saurus  are  examples  of  immense,  quadrupedal  herbivorous  Dino- 
saurs. In  Stegosaurus  the  shortness  of  the  fore  limbs  gives  the 
back  a  very  strong  curvature  ;  this  remarkable  genus  had  a  defen- 
sive armour  of  enormous  bony  plates  and  spines,  extending  in  the 
middle  line  of  the  back  from  the  head  to  the  end  of  the  tail. 
Camptosaurus  was  also  herbivorous,  but  had  an  erect  bipedal 
gait.  Megalosaurus  was  a  carnivorous  reptile,  with  huge  teeth 
and  a  nasal  horn ;  its  fore  legs  are  very  small  and  its  gait  was 
erect.  These  genera  and  others  are  all  found  in  the  Como  beds 
(which  may  be  Jurassic),  and  very  similar  ones  occur  in  the 
Trinity  and  lower  part  of  the  Potomac,  as  also  in  the  Wealden  of 
Europe.  Especially  famous  is  the  genus  Iguanodon,  of  which 
many  complete  skeletons  have  been  found  in  Belgium.  Dino- 
saurs are  much  less  common  in  the  marine  Upper  Cretaceous,  but 
the  green  sands  of  New  Jersey  have  yielded  Hadrosaurus,  an 
herbivorous  Dinosaur  much  like  Iguanodon,  and  some  carnivorous 
types  also.  The  Laramie  and  Denver  beds  have  preserved  many 
fine  specimens,  which  show  that  the  Dinosaurs  flourished  in  almost 


492 


THE  CRETACEOUS   PERIOD 


FIG.  163.  —  Skull  of  Agathaumas  flabellatus,  from  the 
side,  1/30.     (Marsh.) 


undiminished  variety  till  the  end  of  the  Cretaceous.     The  erect, 
herbivorous  type  is   represented  in  these   beds  by  Monoclonius 

and  Diclonius  ( Fig. 
164),  which  are  nearly 
related  to  Hadrosau- 
rus.  A  get  th  a  u  ///  a  s 
(Fig.  163)  and  Toro- 
saurus  are  huge,  quad- 
rupedal reptiles,  with 
three  large  horns  on 
the  head  and  an  ex- 
traordinary frill-like 
extension  of  the  skull 
over  the  neck.  Car- 
nivorous Dinosaurs 
likewise  continued,  such  as  Lalaps  and  Ornithomimus ,  the  latter 
with  hind  limbs  which  are  especially  birdlike  in  structure. 

The  Birds  of  the  Cretaceous  are  much  more  abundant  and 
advanced  than  the  known  Jurassic  birds.  In  the  Upper  Creta- 
ceous of  Kansas,  and 
probably  of  England 
also,  are  found  two 
remarkable  birds,  Hes- 
perornis  and  Ichthy- 
ornis.  In  the  former, 
which  was  nearly  6  feet 
high,  the  wings  were 
rudimentary,  while 
.  Ichthyornis,  a  much  smaller  bird,  had  powerful  wings.  Both  of 
these  genera  possessed  teeth,  like  Archaopteryx,  but  except  in 
that  feature  and  in  certain  minor  details  of  structure,  they  are 
entirely  like  modern  birds.  Bird  bones,  like  the  corresponding 
parts  of  the  Cormorants  and  Waders,  have  been  found  in  the  green 
sands  of  New  Jersey,  but  it  is  not  known  whether  they  had  teeth. 
Mammalia.  —  Cretaceous  Mammals  are  much  more  numerous 
and  varied  than  those  of  the  Jurassic,  but  they  continue  to  play  a 


FIG.  164.  —  Skull  of  Diclonius  mirabilis,  from 
above,  1/19.     (Cope.) 


MAMMALIA 


493 


very  modest  role,  and  are  nearly  all  of  minute  size.  In  America 
they  have  been  found  only  in  the  fresh-water  beds  at  the  base  and 
summit  of  the  Cretaceous,  and  in  Europe  only  in  the  Purbeck  and 
Wealden  at  the  base.  The  Lower  Cretaceous  mammals  differ 
little  from  those  of  the  Jura  (except  for  the  larger  number  of 
genera),  and  from  the  fragmentary  condition  of  the  specimens  it 
is  exceedingly  difficult  to  determine  just  what  groups  are  repre- 
sented. The  Multituberculata  are  believed  to  belong  to  the 
lowest  type  of  mammals,  the  Monotremata,  at  present  represented 
only  by  the  Spiny  Ant-eater  (Echidna)  and  Duck-billed  Mole 
( Ornithorhynchus)  of  Australia.  Of  this  group  the  most  promi- 
nent Lower  Cretaceous  genera  are  the  English  Plagiaulax  and  the 
American  Ctenacodon  and  Allodon.  In  another  group  the  teeth 
are  much  simpler  but  more  numerous  ;  examples  are  Stylodon  and 
Triconodon  from  the  English  Purbeck,  and  Dryolestes  and  Dicro- 
cynodon  from  Wyoming.  In  the  uppermost  Cretaceous  the  mam- 
mals are  much  more  numerous  and  diversified,  and  already 
begin  to  show  affinities  with  the  forms  which  are  to  succeed  them 
in  the  Tertiary.  The  Multituberculata  are  represented  by  two 
genera,  Meniscoessus  and  Ptilodus,  while  other  mammals  of  doubt- 
ful affinities  are  Didelphops,  Pediomys,  and  Cimolestes.  Many 
others  are  known,  but  they  are  too  imperfect  for  reference.  With 
one  exception,  Thlaodon,  which  is  of  moderate  size,  all  these 
mammals  are  exceedingly  small. 

In  brief,  Cretaceous  life  is  still  typically  Mesozoic,  but  a  change 
toward  Cenozoic  conditions  is  already  manifest,  especially  in  the 
Plants,  the  Gastropods,  and  the  Teleostean  Fishes.  There  is  still 
a  gap  between  the  life  systems  of  the  two  eras,  but  it  is  not  so 
wide  as  it  was  once  believed  to  be,  and  it  may  be  hoped  that 
future  discoveries  will  bridge  it  entirely. 


CHAPTER   XXXI 
CENOZOIC  ERA  — TERTIARY  PERIOD 

THE  history  of  the  Cenozoic  era  brings  us  by  gradual  steps  to 
the  present  order  of  things.  Of  no  part  of  geological  history  have 
such  full  and  diversified  records  been  preserved  as  of  the  Ceno- 
zoic, and  yet  this  very  fulness  is  a  source  of  difficulty  and  embar- 
rassment when  we  attempt  to  arrange  the  various  phenomena  in 
their  chronological  order. 

The  sedimentary  rocks  of  the  Cenozoic  era  are,  for  the  most 
part,  quite  loose  and  uncompacted ;  it  is  relatively  rare  to  find 
hard  rocks,  such  as  so  generally  characterize  the  older  formations. 
They  are  also  most  frequently  undisturbed,  retaining  nearly  their 
original  horizontal  positions,  except  when  they  have  been  upturned 
in  the  formation  of  great  mountain  chains.  Another  characteristic 
feature  of  Cenozoic  strata  is  their  locally  restricted  range ;  only 
in  the  oldest  parts  of  the  group  do  we  find  such  widely  extended 
formations  as  are  common  in  the  Palaeozoic  and  Mesozoic  groups, 
and  the  later  Cenozoic  strata  become  more  and  more  local  in  their 
character. 

The  climate  of  the  era  underwent  some  very  remarkable  and 
inexplicable  changes.  At  the  beginning  it  resembled  that  of  the 
Cretaceous  in  its  generally  mild  and  equable  character,  a  luxuriant 
vegetation  flourishing  far  within  the  Arctic  Circle  ;  but  by  very 
slow  gradations  the  climate  grew  colder,  culminating  in  the  Glacial 
Age,  when  much  of  the  land  in  the  Northern  Hemisphere  was 
covered  with  sheets  of  ice  and  snow  and  reduced  to  the  condition 
of  modern  Greenland. 

The  life  of  the  Cenozoic  era  is  very  clearly  demarcated  from 
that  of  the  Mesozoic,  though  many  modern  characteristics  began 
in  the  Cretaceous  or  even  earlier.  The  peculiar  Mesozoic  Am- 

494 


THE  TERTIARY   PERIOD  495 

monoids,  Belemnites,  and  many  curious  Bivalves  disappeared 
almost  entirely  at  the  end  of  the  Cretaceous,  leaving  only  a  few 
stragglers  here  and  there  to  persist  into  the  older  Tertiary.  Even 
more  striking  is  the  dwindling  of  the  Reptiles ;  the  Ichthyosaurs, 
Plesiosaurs,  Pythonomorphs,  Dinosaurs,  and  Pterosaurs,  which  had 
given  such  a  marked  individuality  to  the  Mesozoic  fauna,  have 
become  totally  extinct,  leaving  only  Lizards  and  Snakes,  Turtles 
and  Crocodiles,  to  represent  the  class.  But  Cenozoic  life  is  not 
distinguished  from  Mesozoic  merely  by  negative  characters  ;  it  has 
its  positive  features  as  well.  The  plants  and  invertebrated  ani- 
mals nearly  all  belong  to  genera  which  are  still  living,  and  the 
proportion  of  modern  species  steadily  increases  as  we  approximate 
the  present  time.  The  Fishes,  Amphibia,  and  Reptiles  differ  but 
little  from  those  of  modern  times,  and  the  Birds  take  on  the 
diversity  and  relative  importance  which  characterize  them  now. 
Above  all,  the  Mammals  undergo  a  wonderful  expansion  and  take 
the  place  of  the  vanished  reptiles,  giving  to  Cenozoic  time  an  alto- 
gether different  character  from  all  that  went  before  it.  The  great 
geographical  and  climatic  changes  produced  migrations  of  animals 
and  plants  upon  a  great  scale,  from  continent  to  continent  and 
from  zone  to  zone,  the  result  of  which  is  the  distribution  of  living 
beings  over  the  earth's  surface  as  we  find  it  to-day. 

There  is  some  difference  of  usage  regarding  the  subdivisions 
of  the  Cenozoic  group,  though  the  difference  is  principally  with 
reference  to  the  rank  of  those  subdivisions.  We  shall  follow  the 
usual  American  practice  of  dividing  the  group  into  two  systems, 
the  Tertiary  and  Quaternary. 

THE  TERTIARY  PERIOD 

The  names  Tertiary  and  Quaternary  are  remnants  of  an  old 
geological  nomenclature  which  has  lost  its  significance,  and  was 
proposed  when  the  whole  succession  of  strata  was  believed  to  be 
divisible  into  three  groups,  called  the  Primary,  Secondary,  and  Ter- 
tiary, respectively.  When  it  was  learned  that  there  were  groups 
and  systems  much  older  than  the  so-called  Primary,  the  name  Pa- 


496 


THE  TERTIARY   PERIOD 


Iceozoic  was  substituted  for  Primary,  as  was  Mesozoic  for  Secondary, 
though  the  latter  term  is  still  used,  especially  in  England.  The 
name  Tertiary  has  thus  lost  its  meaning,  but  is  nevertheless 
retained  as  a  division  of  the  Cenozoic  group  or  era. 


Gulf  Border 

Interior  Region 

Pacific  Border 

PLIOCENE 

SERIES 

Floridian  Stage. 

Blanco  Stage. 
Goodnight  Stage. 

PLIOCENE 

(MERCED  SERIES) 

MIOCENE 
SERIES 

Chesapeake  Stage. 
Chipola  Stage. 
Chattahoochee  Stage. 

Loup  Fork  (Nebraska  Substage. 
Stage.      /Deep  River  Substage. 
John  Day  Stage. 

MIOCENE 
("Auriferous 
gravels) 

OLIGOCENE 

? 

White  River  Stage. 

? 

EOCENE 
SERIES 

Upper;    Vicksburg. 

Uinta  Stage. 

TEJON  SERIES 

f  Jackson. 
Middle  \  Claiborne. 
(.  Lower  Claiborne. 

Bridger    f  Bridger  Substage. 
Stage.     (Wind  River  Substage. 
(Green  River) 

Lower  I  LiSnitic- 
(  Midway. 

Wasatch  Stage. 
Puerco.     ?  Fort  Union. 

The  great  revolution  which  closed  the  Cretaceous  and  inaugu- 
rated the  Tertiary  has  left  its  effects  visible  in  all  the  continents, 
but  the  gap  between  the  two  periods  is  not  everywhere  the  same. 
This  revolution  gave  to  North  America  nearly  its  present  outlines, 
in  consequence  of  which  marine  Tertiary  beds  occur  only  along 
the  borders  of  the  continent,  while  the  Tertiary  of  the  interior  is 
all  of  fresh-water  origin.  In  other  continents,  and  especially  in 
Europe,  the  distribution  of  land  and  sea  was  very  different  in  the 
Tertiary  from  what  it  is  now,  and  the  topography  of  the  land  was 
profoundly  altered  in  the  course  of  the  period.  Some  of  the 
highest  mountain  ranges  of  the  earth  were  upheaved  in  Tertiary 
times,  such  as  the  Atlas,  the  Alps,  the  Caucasus,  and  the  Hima- 
layas. That  Tertiary  ranges  are  high  is  not  due  to  any  extreme 
degree  of  compression  as  compared  with  that  which  produced 


EOCENE  497 

older  ranges,  but  merely  to  the  youth  of  the  former ;  denudation 
has  not  yet  had  time  to  sweep  them  away. 

The  Tertiary  system  or  period  is  divisible  into  four  quite  well 
distinguished  series  or  epochs,  which  may  usually  be  identified 
in  both  the  marine  and  fresh-water  formations ;  but  for  lack  of 
common  fossils  it  is  not  yet  possible  to  correlate  the  stages  and 
substages  of  the  interior  region  with  those  of  the  coast.  In  the 
preceding  table,  therefore,  no  exact  comparison  of  these  minor 
subdivisions  is  intended. 

It  has  become  customary  to  distinguish  between  the  older  and 
newer  parts  of  the  Tertiary  by  grouping  together  the  Eocene  and 
Oligocene  into  the  Palaeogene,  and  the  Miocene  and  Pliocene  into 
the  Neogene.  Eocene  and  Neocene  are  employed  in  the  same 
way,  but  this  is  objectionable  because  it  is  using  Eocene  in  two 
different  senses. 

THE  EOCENE  EPOCH 

The  name  Eocene  is  derived  from  two  Greek  words,  —  eos, 
dawn,  and  kainos,  recent,  —  and  was,  like  the  names  of  most  of 
the  other  Tertiary  epochs,  proposed  by  Lyell. 

American.  —  Along  the  Atlantic  and  Gulf  borders  the  coast-line 
of  the  Eocene  closely  follows  that  of  the  Cretaceous,  of  which  only 
a  narrow  strip  separates  the  Eocene  from  the  Triassic  and  crystal- 
line rocks  of  the  Piedmont  plain.  The  unconformity  between  the 
Cretaceous  and  Eocene  indicates  that  along  this  coast  the  latter 
period  had  been  inaugurated  by  an  encroachment  of  the  sea  upon 
the  land.  The  Mississippi  embayment  had  nearly  the  same  size 
and  form  as  before,  extending  up  to  the  mouth  of  the  Ohio. 
Florida  was  entirely  submerged,  as  was  most  of  Central  America, 
cutting  off  the  northern  from  the  southern  continent.  On  the 
Atlantic  coast  the  Eocene  rocks  are  unconsolidated  sands  and 
clays,  with  some  glauconitic  greensand,  particularly  in  New  Jersey. 
They  form  a  narrow  belt  through  New  Jersey,  Maryland,  and  Vir- 
ginia, widening  into  a  quite  broad  band  through  the  Carolinas 
and  the  Gulf  States,  and  extending  around  the  borders  of  the 
Mississippi  embayment  into  Texas.  In  the  Gulf  region  the  rocks 
2  K 


498  THE  TERTIARY   PERIOD 

are  more  consolidated,  and  are  quite  hard  limestones,  sandstones, 
and  shales,  with  extensive  deposits  of  lignite,  formed  in  ancient 
peat  bogs  which  followed  the  low-lying  Gulf  shores. 

On  the  Pacific  coast  a  long,  narrow  arm  of  the  sea  occupied  the 
great  valley  of  California,  extending  northward  into  Oregon  and 
Washington  ;  its  deposits  are  at  present  principally  displayed  along 
the  eastern  flank  of  the  Coast  Range.  These  deposits  form  a 
single  series,  the  Tejon,  which  lies  upon  the  Chico  in  apparent 
conformity;  but  the  lowest  Eocene  is  not  represented  in  the 
Tejon,  and  in  Oregon  an  unconformity  between  the  two  series 
has  been  detected. 

It  was  in  the  interior  region  that  the  geographical  changes 
wrought  by  the  revolution  at  the  end  of  the  Cretaceous  had  the 
most  marked  effects.  Even  before  that  the  movement  of  elevation 
had  converted  the  interior  sea  into  bodies  of  fresh  and  brackish 
water,  in  which  the  latest  Cretaceous  deposits,  the  Laramie  and 
Livingstone,  or  Denver,  had  been  laid  down.  The  same  condi- 
tions appear  to  have  lasted  into  the  Eocene  over  a  part  of  the 
Great  Plains  country.  Covering  much  of  North  Dakota  and  Mon- 
tana and  a  wide  area  in  Canada  was  a  body  of  fresh  water,  in 
t which  were  formed  the  Fort  Union  beds  that  overlie  the  Living- 
stone unconformably.  From  the  evidence  of  the  plants  the  Fort 
Union  is  believed  to  be  the  oldest  Eocene,  but  this  is  still  uncer- 
tain. Fresh-water  beds  with  similar  plants  are  found  in  Greenland 
and  Alaska.  At  the  end  of  Fort  Union  time  the  Great  Plains, 
from  Mexico  to  the  Arctic  Sea,  were  dried  up ;  but  then  began 
the  establishment  of  a  series  of  fresh-water  lakes  in  the  region 
between  the  Wasatch  and  Rocky  Mountain  ranges.  At  present 
this  is  a  region  of  high  plateaus,  elevated  from  5000  to  7000  feet 
above  the  sea,  but  then  it  must  have  been  much  lower  and  can 
hardly  have  been  enclosed  by  such  high  mountains  as  now  encom- 
pass it.  These  lakes  were  not  formed  simultaneously,  but  succes- 
sively, and  together  include  the  whole  of  Eocene  time  in  an  almost 
unbroken  record. 

The  oldest  of  these  lakes  was  the  Piteno,  a  relatively  small  body 
of  water,  which  covered  the  northwestern  part  of  New  Mexico  and 


EOCENE 


499 


the  southwestern  part  of  Colorado.  This  was  followed  by  the  very 
much  larger  Wasatch  lakes,  of  which  there  were  several,  nearly  or 
quite  contemporary.  The  principal  body  of  water  extended  from 
New  Mexico,  over  eastern  Utah  and  western  Colorado  to  the  Uinta 
Mountains,  around  the  eastern  end  of  which  it  formed  a  strait, 
expanding  again  north  of  the  mountains  and  covering  all  south- 
western Wyoming  to  the  Wind  River  Mountains.  This  great  lake 
must  have  been  450  miles  long  by  250  miles  wide  in  its  broadest 
part.  A  second  lake  filled  the  Big  Horn  Basin  of  northwestern 
Wyoming,  which,  then  as  now,  was  shut  in  by  mountains.  In 
southern  Colorado,  east  of  the  main  range  of  the  Rocky  Moun- 
tains, were  two  small  lakes  believed  to  be  of  this  age. 

The  Bridger  lakes,  which  were  much  smaller  than  the  Wasatch, 
were  not  all  contemporaneous,  but  in  part  successive.  The  oldest 
one  (  Wind  River  substage)  occupied  the  Wind  River  Basin,  north 
of  the  mountains  of  that  name.  Two  later  lakes  were  in  the  upper 
Green  River  valley  in  Wyoming  and  a  third  in  the  same  valley 
south  of  the  Uinta  Mountains.  Finally,  a  small  lake  of  this  age 
occupied  the  Huerfano  Canon  in  southern  Colorado. 

A  great  mountain-making  disturbance  drained  the  Bridger  lakes, 
elevating  all  the  ranges  to  which  the  post-Cretaceous  revolution 
had  given  birth  and  establishing  a  new  lake  basin,  the  Uinta.' 
This  basin  lies  principally  to  the  south  of  the  Uinta  Mountains  in 
northeastern  Utah  and  northwestern  Colorado,  and  occupies  part 
of  the  basin  of  the  Wasatch  and  Bridger  lakes.  The  three  stages 
of  strata  may  be  seen  here,  one  over  the  other. 

The  rocks  which  were  accumulated  in  these  successive  lake 
basins  are  principally  sands  and  clays,  with  an  occasional  gravel 
bank.  They  are  indurated  but  still  soft  rocks  which  weather 
readily  and  give  rise  to  the  characteristic  bad-land  scenery  already 
described. 

The  Eocene  epoch  was  brought  to  a  close  by  a  series  of  move- 
ments which  added  a  narrow  strip  of  land  along  the  Atlantic  and 
Gulf  coasts,  at  the  same  time  raising  northern  Florida  into  an 
island.  In  the  interior  the  plateau  region  was  elevated  and 
drained,  and  no  extensive  bodies  of  water  were  ever  established 


500  THE  TERTIARY   PERIOD 

there  again.  Probably  the  upheavals  at  the  end  of  the  Bridger 
and  at  the  end  of  the  Eocene  had  made  the  climate  much  drier, 
by  cutting  off  the  moisture-laden  winds. 

Foreign.  —  The  Old  World  Eocene  has  a  very  different  devel- 
opment from  that  of  North  America,  the  eastern  continents  not 
assuming  their  present  outlines  till  much  later.  At  the  close  of 
the  Cretaceous  period  extensive  geographical  changes  had  taken 
place  in  Europe,  consisting  chiefly  in  the  retreat  of  the  sea  from 
wide  areas  which  it  had  occupied  in  the  Cretaceous.  This  was 
especially  the  case  in  Russia,  northern  Germany  and  France,  and 
southern  England,  and  in  place  of  the  great  gulf  which  had  occu- 
pied these  regions  (see  p.  484)  were  found  only  scattered  bodies 
of  fresh  and  brackish  water.  At  a  later  time  the  sea  again 
advanced  over  part  of  these  areas,  which  explains  the  general  un- 
conformity between  the  Cretaceous  and  Tertiary  strata.  In  south- 
ern Europe  the  Mediterranean  regained  the  great  expansion  which 
it  had  partly  lost  in  the  latter  part  of  the  Cretaceous,  extending 
far  over  northern  Africa,  where  there  is  a  gradual  transition  be- 
tween the  Cretaceous  and  Eocene,  and  transgressing  over  southern 
Europe.  A  long,  narrow  arm  of  this  sea  extended  from  southern 
France,  past  the  north  side  of  the  future  Alps  and  Carpathians, 
into  western  Asia.  Another  narrow  sea,  or  strait,  extended  down 
the  east  side  of  the  Ural  Mountains,  from  the  Arctic  Ocean  to  the 
expanded  Mediterranean,  completely  cutting  off  Europe  from  Asia. 
From  Asia  Minor  the  Mediterranean  extended  across  Persia  and 
Turkistan,  northern  India,  Borneo,  and  Java,  to  the  Pacific,  sepa- 
rating the  southern  peninsulas  from  the  Asiatic  mainland.  There 
was  thus  a  continuous  sea  around  the  earth,  everywhere  separating 
the  southern  continents  from  the  northern. 

In  the  Alpine  and  north  African  regions  were  accumulated  thick 
masses  of  limestone,  largely  composed  of  the  gigantic  foraminiferal 
shells  called  Nummulifes,  but  in  northern  Europe  no  such  widely 
spread  formations  occur.  After  the  Eocene  had  continued  for 
some  time,  a  marine  basin,  the  Anglo-Gallic,  was  formed  over 
southern  England,  northern  France,  and  Belgium,  which  contains 
a  succession  of  alternating  marine,  brackish,  and  fresh-water  strata. 


EOCENE  501 

This  basin  is  classic  ground,  for  in  it  were  made  the  studies  of 
Cuvier  and  Brogniart,  which  led  to  the  recognition  of  the  Tertiary 
as  a  distinct  system  and  founded  the  science  of  Palaeontology. 

On  the  west  coast  of  Africa  the  sea  encroached  in  a  narrow 
belt.  Australia  has  no  marine  Eocene,  but  New  Zealand  has 
extensive  deposits  of  this  epoch,  between  which  and  the  Creta- 
ceous no  definite  line  can  be  drawn. 

The  Tertiary  formations  of  South  America  cannot  yet  be  cor- 
related with  those  of  other  continents,  and  will  be  considered 
together  in  a  separate  section. 

The  Eocene  thus  had  broad  seas  where  now  is  land,  and  con- 
tinents now  connected  were  then  separated  by  straits  and  sounds. 
On  the  other  hand,  there  were  then  land  bridges  joining  land 
areas  which  are  now  far  apart.  Some  of  these  land  bridges  may 
be  reconstructed  with  much  confidence,  while  others  are  more  or 
less  probable.  America  was  connected  with  Asia  across  what  is 
now  Bering's  Sea,  and  also  with  Europe,  probably  by  an  extension 
of  Greenland  and  Iceland.  The  Antarctic  continent  apparently 
had  a  much  greater  extension  than  it  has  now,  and  seems  to  have 
been  joined  with  both  Australia  and  South  America.  It  is  quite 
possible  that  Africa  was  more  or  less  directly  connected  with  the 
same  land  mass.  If  this  be  true,  then  in  Eocene  times  the  north- 
ern continents,  Europe  and  Asia,  were  joined  in  the  Arctic  latitudes 
by  way  of  North  America,  while  South  America,  Africa,  and  Aus- 
tralia radiated  in  three  great  lines  from  the  South  Pole.  Between 
the  two  series  of  continents,  northern  and  southern,  swept  the 
transverse  seas,  of  which  the  Mediterranean  and  Caribbean  are 
remnants. 

Eocene  Life 

Except  for  the  Vertebrates,  Eocene  life  is  chiefly  instructive 
from  the  manner  of  its  distribution  over  the  globe.  Invertebrates 
and  plants  are  nearly  the  same  as  modern  forms,  the  genera,  for 
the  most  part,  still  existing,  though  the  species  are  nearly  all 
extinct. 

Plants. — The  Eocene  flora  of  North  America  is  found  pre- 
served in  widely  separated  localities,  —  Canada,  Montana,  Wyo- 


502 


THE  TERTIARY   PERIOD 


ming,  and  Idaho.  It  was  very  rich  and  varied,  and  bears  evidence 
of  a  climate  much  milder  than  now  obtains  in  those  localities. 
Besides  Ferns  and  Horsetails,  this  flora  includes  some  Grasses, 
Bananas,  and  many  noble  Palms  (Fig.  165),  Myrtles,  Beeches, 
Oaks,  Willows,  Poplars,  Elms,  Sycamores,  Laurels,  Magnolias, 
Maples,  Walnuts,  Pines,  Spruces,  Arbor  Vitae,  and  the  like.  Even 

in  Greenland  and 
Alaska  was  a  lux- 
uriant growth  of 
forests  of  a  tem- 
perate character, 
such  as  could  not 
exist  there  now. 

The  European 
flora  has  a  more 
decidedly  tropical 
character  than  that 
of  North  Amer- 
ica, and  contains 
plants  whose  near- 
est living  allies  are 
now  widely  scat- 
tered, occurring  in 
the  warmer  parts 
of  America,  Africa,  Asia,  and  Australia.  Cypresses,  Yews,  and 
Pines  are  abundant,  including  the  Sequoia,  now  confined  to 
California,  and  the  Gingko  of  China  and  Japan.  Aloes,  Palms, 
and  Screw-pines  occur,  mingled  with  the  ordinary  temperate  forest 
trees,  Elms,  Poplars,  Willows,  Oaks,  etc.  The  distribution  of  plants 
in  the  Eocene  was  thus  very  different  from  what  it  is  at  present. 

Animals.  —  Foraminifera  of  relatively  enormous  size  abounded, 
and  their  shells  make  up  great  rock  masses.  Orbitolites  is  a  con- 
spicuous genus  along  our  Gulf  coasts,  Nummulites  in  the  Old  World. 
Corals  are  completely  modern  in  character.  The  Sea-urchins  and 
especially  the  Irregulares  are  much  the  most  important  repre- 
sentatives of  the  Echinoderms.  Of  the  Mollusca  both  Bivalves 


FIG.    165.  —  Flabellaria  eocenica,  1/12. 
Shales. 


Green  River 


EOCENE  503 

(PI.  XI,  Figs.  2,  3)  and  Gastropods  (XI,  4,  5)  increase  greatly  and 
are  very  rich  in  species.  Nautiloid  Cephalopods  are  more  varied 
and  widely  distributed  than  now  (XI,  8),  and  in  a  few  localities 
Ammonites  and  Belemnites  have  been  found,  but  these  are  mere 
belated  stragglers  from  the  Cretaceous  and  are  much  too  rare  to 
be  at  all  characteristic.  Among  the  Crustacea  should  be  noted 
the  great  increase  of  the  Crabs,  which  are  even  more  numerous 
and  varied  than  in  the  Cretaceous. 

The  Fishes,  both  fresh-water  and  marine,  differ  only  in  minor 
details  from  modern  fishes.  The  Reptiles  are  likewise  essentially 
modern  in  character,  and  only  two  groups,  the  Lizards  and  Snakes, 
are  more  numerous  than  they  had  been  in  Mesozoic  times,  though 
the  venomous  snakes  had  not  yet  appeared.  The  Eocene  lakes 
of  the  West  contained  multitudes  of  large  Crocodiles  and  a  great 
variety  of  Turtles. 

Eocene  Birds  are  very  much  more  numerous,  advanced,  and 
diversified  than  those  of  the  Cretaceous  ;  one  characteristic  feature 
of  the  times  was  the  presence  in  Europe  and  America  of  extremely 
large,  flightless  birds,  more  or  less  like  the  ostriches  in  appearance. 
Of  flying  birds  there  were  many  kinds ;  Owls,  Eagles,  Buzzards, 
Vultures,  Gulls,  Waders,  Woodcock,  Quail,  Ibis,  and  Pelicans  are 
represented  by  ancestral  forms,  somewhat  different  from  their 
modern  descendants. 

The  Mammals  have  developed  in  a  marvellous  way  since  the 
Cretaceous,  assuming  in  terrestrial  life  that  dominant  place  which 
they  have  ever  since  held.  Compared  with  the  evolution  of  other 
animal  groups,  that  of  the  mammals  has  been  so  rapid  that  each 
stage  of  the  Eocene  has  its  own  mammalian  fauna,  differing  from 
those  of  the  preceding  and  succeeding  stages.  Besides  these 
geological  differences  between  the  successive  mammalian  assem- 
blages, there  are  often  marked  geographical  differences  between 
the  faunas  which  are  of  approximately  contemporaneous  age, 
but  widely  separated  in  space.  Comparing  Europe  and  North 
America  in  this  respect,  we  find  .that  in  the  Eocene  each  con- 
tinent had  its  own  peculiarities,  but  that  the  land  connection  be- 
tween them  allowed  continual  intermigration  and  thus  kept  up 


504 


THE  TERTIARY   PERIOD 


PLATE  XI.   AMERICAN  TERTIARY  FOSSILS 


EOCENE  505 

a  close  general  similarity  in  their  mammals.  The  southern  conti- 
nents, on  the  other  hand,  had  altogether  different  mammalian 
faunas,  due  to  their  long  separation  from  the  northern  lands. 

The  PUERCO  shows  its  close  relations  with  the  Mesozoic  in  the 
presence  of  numerous  Multituberculata,  the  last  and  largest  of 
that  group.  (Ptilodus  and  Polymastodon  are  the  common  genera.) 
The  primitive  type  of  flesh-eaters  (Creadon fa)  and  ancestors  of 
the  true  Carnivores  are  abundant,  as  are  also  the  primitive  hoofed 
animals  ( Condylarthra  and  Amblypoda)  ;  the  curious  Tittodonts, 
Ganodonts,  and  primaeval  Lemuroids  complete  the  list.  Espe- 
cially noteworthy  is  the  entire  absence  of  Rodents,  of  true  Carni- 
vores, of  Artiodactyls,  and  Perissodactyls. 

The  change  to  the  WASATCH  is  very  abrupt,  and  was  probably 
due  to  a  great  migration  of  mammals  from  some  region,  as  yet 
unknown.  Rodents  come  in  for  the  first  time  in  North  America. 
Perissodactyls  make  their  first  appearance  with  ancestral  members 
of  the  horse  family  (Hyracotheriuni],  of  the  rhinoceroses  (Hepto- 
don),  the  tapirs  (Sysfemodon) ,  and  other  families  now  extinct. 
The  curious  extinct  group  of  hoofed  animals  called  the  Ambly- 
poda greatly  increases  in  numbers  and  in  stature,  and  both  in 
Europe  and  America  the  predominant  genus  is  Coryphodon. 
Artiodactyls  also  appear  for  the  first  time  in  ancestral  members 
of  the  pigs  (Eohyus),  and  the  ruminants  (Trigonolestes).  The 
Creodonts  increase  in  numbers  and  in  the  size  and  strength  of  the 
individuals,. /Szi^jYzsza  being  as  large  as  a  bear,  and  Oxycena  was 
an  aquatic  form.  Numerous  Lemuroids  and  primitive  types  of 
Monkeys  (Anaptomorphus)  swarmed  in  the  trees.  The  corre- 
spondence between  the  mammals  of  Europe  and  North  America 
was  never  closer  than  in  Wasatch  times. 

Between  the  Wasatch  and  Bridger  lie  the  GREEN  RIVER  SHALES, 

EXPLANATION  OF  PLATE  XI,  p.  504.  i.  Ostrea  virginiana,  1/2.  Miocene. 
(Whitfield.)  2.  Pecten  madisonicus,  1/2,  Miocene.  (Whitfield.)  3.  Cardita  per- 
antiqua,  Eocene.  (Whitfield.)  4.  Volutolithes  sayana,  3/4,  Eocene.  (Whitfield.) 
5.  Oliva  carolinensis,  3/4,  Miocene.  (Whitfield.)  6.  Helix  Dalli,  Miocene. 
(White.)  7.  Planorbis  convoluta,  ?  Fort  Union.  (Meek.)  8.  Aturia  Vanuxemi, 
1/4,  Eocene.  (Whitfield.)  9.  Glyptostrobus  Ungeri,  1/2,  Eocene.  (Lesquereux.) 
10.  Salix  sp.,  3/4,  Oligocene. 


506  THE  TERTIARY   PERIOD 

which  are  believed  to  represent  the  lower  Bridger  (  Wind  River 
substage)  in  time.  These  shales  have  yielded  no  mammalian 
remains,  but  great  numbers  of  plants,  insects,  and  fishes  occur  in 
them. 

The  BRIDGER  mammals  represent  a  steady  advance  upon  those 
of  the  Wasatch,  but  there  is  no  such  complete  change  as  followed 
the  Puerco.  The  Perissodactyls  may  be  said  to  culminate  in  the 
Bridger ;  for  though  they  afterwards  reached  much  higher  stages 
of  development,  they  never  again  had  the  same  relative  impor- 
tance. Horses,  Tapirs,  Rhinoceroses,  and  Titanotheres  (Palao- 
syops)  are  extraordinarily  abundant.  Coryphodon  has  vanished, 
but  the  wonderful  elephantine,  six-horned  Uintatherium  takes  its 
place  in  North  America,  though  not  in  Europe.  Artiodactyls, 
Creodonts,  Rodents,  Tillodonts,  and  Lemurs  were  more  diver- 
sified than  ever,  and  Bats  are  found  here  for  the  first  time. 

In  the  UINTA  the  mammals  continue  to  advance  along  the  same 
lines,  gaining  in  size  and  advancing  in  structure.  Noteworthy 
is  the  rppearance  of  the  true  Carnivores  (Cynodictis),  and  the 
great  development  of  the  Artiodactyls,  which  in  Europe  display 
wonderfully  manifold  types  of  structure.  In  America  Oreodonts 
(Protoreodon)  and  primitive  Camels  (Leptotragulus]  are  the  com- 
monest forms.  Creodonts,  Lemurs,  and  Tillodonts  decline. 

In  the  Upper  Eocene  seas  great  whale-like  forms  (Zeuglodon) 
of  extraordinary  appearance  and  structure  had  grown  abundant. 

Volcanic  eruptions  continued  in  the  Rocky  Mountain  region 
during  the  Eocene.  The  Yellowstone  Park  was  a  centre  of  great 
activity,  with  numerous  cones  ejecting  acid  lavas  and  tuffs. 

The  climate  of  the  Eocene  was  very  much  the  same  as  in  the 
Cretaceous,  mild  and  equable  all  over  the  Northern  Hemisphere, 
at  least,  as  is  shown  by  the  character  of  the  vegetation  and  the 
marine  shells,  though  probably  there  was  already  a  beginning  of 
climatic  zones. 

THE  OLIGOCENE  EPOCH 

American. — This  term  (derived  from  the  Greek  oligos,  little, 
and  kainos,  recent)  is  seldom  used  in  this  country,  but  it  is  impor- 


OLIGOCENE  507 

tant  to  follow  the  European  scale,  wherever  that  is  practicable. 
No  marine  beds  are  yet  known  in  North  America  belonging  to 
this  series,  but  in  the  interior  region  are  extensive  fresh-water 
deposits  which  clearly  should  be  referred  to  it,  and  which  form 
the  WHITE  RIVER  stage.  The  largest  body  of  water  of  this  time 
occupied  northeastern  Colorado,  southeastern  Wyoming,  much  of 
western  Nebraska,  and  South  Dakota.  A  second  lake  of  unknown 
extent  was  in  North  Dakota,  and  may  have  been  joined  with  the 
first ;  the  present  severance  of  the  deposits  may  be  due  to  the 
removal  of  the  strata  over  the  intervening  territory.  A  third  and 
much  smaller  area  is  in  the  Cypress  Hills  of  the  Canadian  North- 
west Territory.  The  strata  formed  in  these  waters,  partly  fluvia- 
tile  and  partly  lacustrine  deposits,  are  almost  perfectly  horizontal, 
and  consist  of  pale- coloured,  moderately  indurated  sands  and  clays, 
which  weather  into  fantastic  bad  lands  (see  Figs.  23,  24,  pp. 
79,  80;  Fig.  133,  p.  .317).  In  the  South  Park  of  Colorado,  at 
Florissant,  was  a  very  small  lake,  probably  of  Oligocene  date,  in 
which  showers  of  fine  volcanic  ashes  formed  thin,  papery  shales, 
which  have  preserved  great  numbers  of  plants  and  insects. 

No  very  marked  disturbance  closed  the  White  River  age ;  the 
lakes  were  simply  filled  or  drained  and  deposition  ceased. 

Foreign.  —  During  the  Eocene  nearly  all  Germany  had  been 
land,  but  in  the  Oligocene  it  was  invaded  by  the  sea,  which  broke 
in  from  the  north  and  covered  all  the  northern  plain,  extending 
into  Belgium,  and  sending  long  bays  to  the  south.  One  of  these 
reached  to  the  strait  on  the  north  of  the  Alps,  expanding  into 
a  large  basin  near  Mayence  and  Frankfort.  Over  Germany  the 
sea  was  shallow,  permitting  the  formation  of  extensive  peat  bogs, 
where  were  accumulated  masses  of  lignite  or  brown  coal.  In 
the  basin  of  Paris  the  sea  had  a  greater  extent  than  in  Eocene 
times,  though  with  lacustrine  beds  intercalated,  but  in  England  the 
beds  are  more  of  brackish-  and  fresh-water  origin.  In  southern 
Europe  the  sea  retreated  from  wide  areas,  and  in  its  place  were 
extensive  bodies  of  fresh  and  brackish  water,  in  many  of  which 
peat  bogs  accumulated  masses  of  lignite.  Such  lignitic  deposits 
occur  at  intervals  from  the  south  of  France,  Switzerland;  and 


508  THE  TERTIARY  PERIOD 

Bavaria,  as  far  east  as  Hungary  and  Dalmatia.     The  Ural  Sea  was 
drained  and  Europe  united  with  Asia. 

Oligocene  Life 

Plants.  —  The  shales  of  Florissant  have  yielded  a  very  rich 
flora,  which  resembles  that  of  the  Eocene  in  general  character, 
though  with  some  marked  differences.  One  such  difference  is 
the  abundance  of  Conifers  at  Florissant,  among  them  Sequoia, 
which  have  not  been  found  in  the  American  Eocene,  and  another 
is  the  great  rarity  of  Palms.  The  flora  is  of  a  warm  temperate, 
but  not  at  all  subtropical  character.  In  Europe  the  Oligocene 
vegetation  of  the  south  is  still  subtropical,  and  contains  an  increas- 
ing number  of  plants  like  those  which  at  present  inhabit  the  warmer 
parts  of  North  America.  In  northern  Europe  there  is  a  change  in 
the  flora,  palms  being  less  common  than  they  had  been  before. 

Animals.  —  The  Insects  of  Florissant  are  very  numerous,  and 
differ  little  from  modern  forms.  They,  like  the  plants  and  the 
Fishes  of  the  same  locality,  resemble  those  now  found  in  the 
warmer  parts  of  the  United  States. 

The  White  River  beds  contain  a  wonderfully  rich  and  varied 
vertebrate  fauna,  and  one  which  is  very  closely  related  to  the 
contemporary  fauna  of  Europe.  The  Reptiles  show  a  great 
change  from  the  Eocene ;  the  large  Crocodiles,  which  were  so 
common,  have  vanished  from  the  northern  interior,  and  only 
one  small  and  rare  species  has  been  found.  The  Turtles  are 
still  numerous,  but  not  nearly  so  varied  as  in  the  Eocene. 

Mammals  have  been  preserved  in  astonishing  numbers,  and 
though  they  are  much  like  those  of  the  Uinta,  they  show  great 
progress  since  that  time.  The  true  Carnivores  now  become  abun- 
dant, represented  by  primitive  Dogs,  Sabre-tooth  Cats,  and  Wea- 
sels, the  latter  family  much  more  numerous  in  Europe,  where 
also  occur  Civet-cats,  a  family  that  never  reached  America.  The 
true  Carnivores  have  displaced  the  Creodonts,  which  have  all  died 
out,  except  two  curious  genera,  Hyanodon  and  Pterodon.  The 
Lemurs  and  Monkeys  (with  one  doubtful  exception)  have  also 
disappeared  from  North  America.  Perissodactyls  are  still  very 


OLIGOCENE  509 

abundant;  the  Horses  are  represented  by  the  little  three-toed 
Mesohippus  (Fig.  166),  the  Tapirs  by  Protapirus,  and  the  Rhinoce- 
roses by  many  types,  all  of  them  hornless ;  thus  Metamynodon 
was  a  heavy,  aquatic  animal,  somewhat  like  a  hippopotamus  in 
appearance ;  Ccenopus  was  a  stout  terrestrial  form,  and  Hyraco- 
don  (Fig.  167)  a  long-legged,  slender,  running  animal.  The 
Titanotheres  culminate  in  the  huge  Titanotherium,  which  had  a 
pair  of  long  horns  on  the  nose.  Artiodactyls  are  now  much  more 
numerous  than  the  Perissodactyls,  but  are  quite  different  in  the 


FIG.  166.  —  Skeleton  of  Mesohippus  Bairdi.    (Fair.) 

two  continents.  Besides  a  number  of  curious  extinct  types  com- 
mon to  the  two  regions,  such  as  Ancodus,  Anthracotherium, 
Elotherium,  America  has  Oreodonts,  Camels  (Pocbrotherium) , 
Peccaries  (Perchcerus),  and  the  extraordinary  little  Protoceras, 
while  Europe  had  a  remarkable  variety  of  Xiphodonts,  Anoplo- 
theres,  true  Swine,  and  true  Ruminants.  The  Rodents  of  the 
White  River  stage  are  much  more  numerous  and  varied  than 
before.  Marmots,  Squirrels,  Beavers,  Mice,  Pocket-gophers,  and 
Rabbits  are  already  well  established. 

From  the  change  in  the  character  of  the  vegetation  and  of  the 
reptiles  we  may  infer  that  a  slight  change  had  taken  place  in  the 
climate,  which  was  not  quite  so  warm  as  before,  at  lea^TuTthe  in- 
terior of  the  continent.  A  similar  change  occurred  in  northern 
Europe  in  the  later  Oligocene. 


5io 


THE  TERTIARY   PERIOD 


MIOCENE  5  1 1 

THE  MIOCENE  EPOCH 

American.  —  At  the  opening  of  the  Miocene,  the  coast-line  of 
the  Atlantic  and  Gulf  occupied  nearly  the  same  position  as  at  the 
beginning  of  the  Eocene,  differing  only  by  the  presence  of  a  nar- 
row strip  of  coast  and  of  the  Florida  island,  which  had  been  pro- 
duced by  the  slight  movements  during  or  at  the  end  of  the  Eocene. 
The  whole  thickness  of  the  Miocene  strata  is  not  found  everywhere 
along  the  coast ;  in  Maryland  and  Virginia  a  slight  transgression  of 
the  sea  occurred,  and  Upper  Miocene  beds  were  deposited  upon 
Lower  Eocene.  Miocene  beds  occur  in  Martha's  Vineyard,  are 
apparently  concealed  beneath  the  sea  along  the  New  England 
coast,  and  are  continuous  southward  from  New  Jersey.  In  that 
state  they  have  a  thickness  of  700  feet,  thinning  to  400  feet  in 
Maryland,  but  attaining  a  thickness  of  1500  feet  in  Texas.  In  the 
north  the  strata  are  unconsolidated  clays  and  sands,  but  in  Florida 
they  are  largely  compacted  limestones,  and  in  Georgia,  limestones 
and  conglomerates.  The  Mississippi  embayment  was  much  nar- 
rowed by  the  Eocene  uplift,  and  Miocene  strata  have  not  been 
found  in  Tennessee  or  Arkansas ;  in  eastern  Texas  they  are  cov- 
ered by  newer  deposits,  but  their  presence  is  revealed  by  deep 
borings. 

The  older  Miocene  of  the  Atlantic  and  Gulf  (  Chattahoochee  and 
Chipola  stages)  had  a  warm-water  fauna  very  similar  to  that  of  the 
West  Indies  and  Central  America ;  some  of  the  West  Indian  spe- 
cies ranged  as  far  north  as  New  Jersey.  The  newer  Miocene,  on 
the  other  hand  {Chesapeake  stage),  shows  a  very  marked  faunal 
change  which  points  to  the  influx  of  cooler  waters.  This  change 
was  probably  due  to  the  increasing  size  and  elevation  of  the  Florida 
island,  and  the  formation  of  the  "  Carolina  ridge  "  in  the  bed  of 
the  Atlantic,  which  diverted  the  Gulf  Stream,  pushing  it  farther 
away  from  the  coast  than  it  had  been  before,  or  than  it  is  now. 
This  allowed  a  cooler  current  from  the  north  to  follow  the  shore  all 
the  way  down  to  the  Gulf  of  Mexico.  Central  America  was  still, 
for  the  most  part,  under  water,  and  rocks  of  Miocene  age  make 
up  most  of  the  interior  mountain  ranges  of  Costa  Rica.  Miocene 


512  THE  TERTIARY   PERIOD 

strata,  chiefly  limestones,  are  also  quite  extensively  developed  in 
the  West  Indian  islands. 

In  California  an  elevation  at  the  end  of  the  Eocene,  or  early  in 
the  Miocene,  had  shifted  the  shore-line  far  to  the  west.  The  late 
Cretaceous  peneplain  of  the  Sierra  foothills  had  been  elevated 
and  carved  into  ridges.  In  the  lower  stream  courses  and  the 
deepest  parts  of  the  valleys  some  heavy  gravels  had  accumulated 
(the  deep  Auriferous  Gravels).  In  the  Upper  Miocene  the  sea 
again  advanced,  raising  the  shore-line  420  feet  above  the  present 
sea-level,  depositing  sediments  (the  lone  stage)  along  the  foothills 
of  the  Sierra.  In  the  stream  valleys  very  thick  masses  of  gravel 
were  accumulated  (the  bench  Atiriferous  Gravels).  After  the 
deposition  of  the  gravels  came  a  time  of  great  volcanic  activity  in 
the  Sierra,  first,  of  rhyolite  flows  accompanied  by  sheets  of  tuff, 
and,  after  an  interval,  of  andesite  tuffs  and  breccias,  which  poured 
down  the  valleys  as  immense  torrents  of  mud.  Marine  Miocene 
beds  are  found  on  both  sides  of  the  Coast  Range,  which  then 
formed  a  chain  of  reefs  and  islands.  Into  the  northern  part  of 
the  Sacramento  valley,  the  sea  did  not  extend,  this  portion  being 
occupied  by  a  lake  which  is  supposed  to  be  Miocene.  The  coast 
of  Washington  and  Oregon,  including  much  of  the  Coast  Range 
in  the  latter  state,  was  covered  by  the  sea,  which  extended  up  the 
valley  of  the  Columbia  and  that  of  its  southern  tributary,  the 
Willamette.  Puget  Sound  was  broader  than  now,  and  more  widely 
connected  with  the  Pacific ;  indeed,  the  whole  peninsula  between 
the  sound  and  the  ocean,  with  the  Olympic  Range,  may  have  been 
submerged.  The  sea  also  extended  over  parts  of  the  British 
Columbian  coast.  Alaska  was  depressed  in  the  early  Miocene, 
especially  toward  the  north,  and  the  valley  of  the  Yukon  was  in- 
vaded by  the  ocean ;  probably  the  eastern  part  of  Bering's  Sea, 
and  much  of  the  lowlands  of  western  Alaska,  were  covered  with 
shallow  water. 

In  the  interior  region  are  extensive  fresh-water  deposits  of 
Miocene  date.  The  oldest  of  these  form  the  JOHN  DAY  stage  of 
eastern  Oregon,  which  spreads  between  the  Cascade  and  Blue 
Mountains,  and  southward  may  extend  into  Nevada.  The  thick- 


MIOCENE  5  1 3 

ness  of  these  beds  is  from  3000  to  4000  feet,  and  they  are  largely 
composed  of  stratified  volcanic  ashes  and  tuffs,  which  were  show- 
ered into  the  lake  by  vents  not  very  far  away,  probably  in  the 
Cascades.  A  second  and  much  smaller  lake  of  this  age  filled  the 
central  valley  of  Montana.  The  John  Day  age  must  have  followed 
the  White  River  after  an  interval  which,  geologically  speaking,  was 
quite  short,  and  it  seems  arbitrary  to  refer  the  two  stages  to 
different  epochs. 

After  the  John  Day  lakes  had  been  obliterated,  no  deposits  were 
made  in  the  interior  region  for  a  considerable  period,  and,  so  far 
as  is  yet  known,  the  Middle  Miocene  is  not  represented  in  that 
region,  but  in  the  Upper  Miocene  these  bodies  of  water  were  rees- 
tablished on  a  greater  scale  than  ever.  The  LOUP  FORK  stage  is 
composed  of  two  substages,  one  of  which  is  quite  distinctly  older 
than  the  other,  and  has  a  much  more  restricted  range.  This  first 
substage,  the  Deep  River,  was  formed  in  a  lake,  or  several  of  them, 
which  filled  the  mountain  basins  of  central  and  southern  Montana, 
north  of  the  Yellowstone  Park.  The  second  substage,  or  Nebraska, 
is  the  most  widely  distributed  of  all  the  fresh-water  Tertiary  de- 
posits of  the  interior,  and  is  partly  of  lake,  partly  of  river  origin. 
The  beds  extend  from  South  Dakota  far  into  Mexico.  Another 
area  of  these  beds  is  in  eastern  Oregon,  where  they  unconform- 
ably  overlie  the  John  Day.  The  Loup  Fork  is  much  thinner  than 
the  John  Day  and  White  River,  and  its  rocks,  sands,  grits,  and 
marls  are  scarcely  at  all  indurated,  not  forming  bad  lands. 

Besides  these  deposits  whose  geological  position  is  established 
by  the  numerous  fossils  which  they  contain,  there  are  several  others 
which  are  referred  to  the  Miocene,  though  with  much  doubt.  One 
such  lake  occupied  the  upper  Sacramento  valley  in  California,  and 
stretched  far  to  the  northeast  through  the  narrow  strait  between 
the  Sierra  and  the  Coast  Range ;  another  was  in  western  Nevada. 
Others  again  occupied  valleys  in  the  interior  of  British  Columbia. 

The  Miocene  was  a  time  of  great  volcanic  activity  among  the 

mountain  ranges  of  the  Pacific  coast,  and  along  the  main  range  of 

the  Rocky  Mountains ;  the  great  volcanoes  of  Mexico  and  of  the 

Cascade  Mountains  are  believed  to  date  from  this  epoch.     The 

2  L 


514  THE  TERTIARY   PERIOD 

Yellowstone  Park  continued  to  be  an  active  volcanic  centre,  and 
was  characterized  by  great  ejections  of  basic  andesites  and  basalts, 
both  lavas  and  tuffs. 

The  close  of  the  Miocene  in  North  America  was  marked  by 
some  extensive  geographical  changes.  Central  America  and  the 
Isthmus  were  upheaved,  joining  North  and  South  America,  and 
cutting  off  the  connection  between  the  Atlantic  and  Pacific.  The 
effects  of  this  land  bridge  were  soon  made  evident  in  the  intermi- 
grations  of  the  land  mammals  which  had  been  peculiar  to  one  or 
other  continent,  South  American  types  appearing  in  North  Amer- 
ica and  vice  versa.  The  same  movement  elevated  the  West  Indian 
islands,  and  joined  Florida  to  the  mainland,  adding  at  the  same 
time  a  narrow  belt  to  the  Atlantic  and  Gulf  coasts.  Along  the 
Pacific  coast  an  upturning  of  the  strata  and  elevation  of  the  moun- 
tain ranges  took  place,  though  subsequent  movements  added  much 
to  the  height  of  the  mountains. 

Foreign.  —  In  the  north  of  Europe  the  sea  retreated  from  large 
areas ;  northern  Germany  was  now  dry  land,  with  only  a  relatively 
small  bay  invading  it,  while  England  was  entirely  above  water,  and 
has  no  marine  Miocene  beds.  On  the  west  coast  of  Europe,  the 
Atlantic  encroached  largely,  as  in  France,  Spain,  Portugal,  and  also 
the  northwest  of  Africa.  Spain  was  joined  to  Africa,  but  straits 
across  northern  Spain  and  southern  France  connected  the  Atlantic 
with  the  Mediterranean.  Another  change  of  great  importance  was 
the  shutting  off  of  the  long-standing  connection  of  the  Mediter- 
ranean with  the  Indian  seas.  The  former  covered  much  of  east- 
ern Spain,  and  flooded  the  lower  Rhone  valley,  sending  an  arm 
along  the  northern  border  of  the  Alps  to  the  neighbourhood  of 
Vienna.  Here  it  expanded  into  a  broad  basin,  connected  with 
another  great  basin  covering  Hungary.  Most  of  Italy,  Sicily,  and 
a  large  part  of  northern  Asia  Minor  were  under  water,  but  the 
Adriatic  and  ^gean  Seas  were  mostly  land,  and  the  Alps  formed  a 
chain  of  islands,  mountainous,  but  not  nearly  so  high  as  at  present. 

At  the  end  of  the  Lower  Miocene  came  a  great  upheaval  of  the 
Alps,  by  which  the  sea  was  again  excluded  from  that  region,  and, 
just  as  in  the  Oligocene,  inland  seas  and  lakes  took  the  place  ot 


MIOCENE  5  1 5 

the  marine  straits.  The  basins  of  Vienna  and  Hungary  had  a  very 
complex  history,  with  repeated  changes  of  size  and  position,  re- 
sulting in  the  formation  of  an  immense  inland  sea  (the  Sarmatian 
Sea),  which  reached  from  Vienna  to  the  Black,  Aral,  and  ^Egean 
Seas,  and  was  nearly  as  large  as  the  present  Mediterranean.  This 
vast  basin  had  but  a  limited  connection  with  the  ocean,  and  repre- 
sented conditions  much  like  those  of  the  Black  Sea  at  present. 
Europe  had  also  a  number  of  fresh-water  lakes,  particularly  in 
France,  Switzerland,  and  Germany,  which  have  preserved  a  very 
interesting  record  of  Miocene  land  life.  A  comparison  with  that 
of  North  America  shows  that  a  way  of  migration  was  still  open 
between  the  two  continents. 

In  the  Old  World  the  Miocene  was  a  time  of  mountain  making. 
The  Pyrenees  had  been  elevated  in  the  later  Eocene ;  the  Alps 
received  nearly  their  present  altitude  in  the  Miocene.  The  Apen- 
nines had  two  distinct  phases  of  upheaval,  one  in  the  Eocene  and 
one  in  the  Miocene,  the  latter  coinciding  with  that  of  the  Alps.  The 
CaucasusMates  from  the  close  of  the  Miocene,  while  the  date  of  the 
Himalayas,  is  yet  uncertain,  but  was  either  Eocene  or  Miocene. 

Marine  Miocene  beds  occur  in  northeast  Africa,  on  the  coast  of 
the  Soudan,  and  in  Australia  and  New  Zealand. 

Miocene  Life 

The  life  of  the  Miocene  is  in  all  respects  a  great  advance  upon 
that  of  the  Eocene  and  Oligocene.  The  Grasses  greatly  multiply 
and  take  possession  of  the  open  spaces,  producing  a  revolution 
in  the  conditions  of  food  for  the  herbivorous  animals.  The  vege- 
tation of  North  America,  as  far  north  as  Montana,  perhaps  even 
to  northern  British  Columbia,  still  bore  a  southern  character.  In 
the  Upper  Miocene  tuffs  of  the  Yellowstone  Park  and  contemporary 
strata  of  Oregon  are  found  such  trees  as  Poplars,  Walnuts,  Hicko- 
ries, Oaks,  Elms,  Maples,  Beeches,  noble  forms  of  Magnolias  and 
Sycamores.  One  species  of  Aralia  had  leaves  2  feet  long  by  3 
inches  wide.  Curiously  enough,  the  Breadfruit  (Artoc&rpus)  flour- 
ished in  Oregon,  and  probably  on  the  Yellowstone  also.  Conifers 
were  numerous  and  varied. 


516  THE  TERTIARY   PERIOD 

In  Europe  the  lower  Miocene  flora  was  quite  like  that  of  modern 
India;  over  the  central  and  western  regions  Palms  continue  to 
flourish,  together  with  Live  Oaks,  Myrtles,  Magnolias,  Figs,  etc. 
In  the  latter  part  of  the  epoch  a  change  is  noted,  and  such  trees 
as  Beeches,  Poplars,  Elms,  Maples,  Laurels,  and  the  like  become 
dominant. 

Marine  Invertebrates  belong  almost  entirely  to  genera  which 
still  live  in  the  seas,  and  many  of  the  species  persist  to  our  own 
day.  Both  in  North  America  and  Europe  the  older  Miocene  has 
numbers  of  shells  such  as  now  live  only  in  warm  seas,  like  Cyprcea, 
Mitra,  Murex,  Strombus,  etc.  (See  PI.  XII.)  The  newer  Mio- 
cene of  our  Atlantic  coast  was  evidently  a  time  of  cooler  waters, 
and  a  similar  change  took  place  in  Europe. 

The  terrestrial  Vertebrates  of  the  interior  are  of  much  interest. 
Little  is  known  of  Miocene  Birds  in  this  country,  but  in  Europe 
they  are  abundantly  preserved  and  are  of  distinctly  African 
character.  Parrots,  Indian  Swallows,  Secretary  Birds,  Adjutants, 
Cranes,  Flamingoes,  Ibises,  Pelicans,  Sand-grouse,  and  numerous 
Gallinaceous  birds,  were  mingled  with  birds  of  European  type,  such 
as  Eagles,  Owls,  Woodpeckers,  Gulls,  Ducks,  etc. 

The  Mammals  of  the  John  Day  are  much  like  those  of  the 
White  River  Oligocene,  but  are  more  modernized  and  advanced. 
Ancient  types,  like  the  Creodonts,  Anthracotheres,  Titanotheres, 
aquatic  and  cursorial  Rhinoceroses,  have  died  out,  while  the  Car- 
nivores and  Rodents  greatly  increase  in  numbers  and  variety.  In 
Europe  the  Lower  Miocene  mammals  are  very  similar  to  those  of 
North  America,  but  one  marked  difference  is  in  the  profusion  of 
true  Ruminants,  of  which  the  western  continent  had  none. 

Between  the  John  Day  and  the  Loup  Fork  is  a  long  gap,  as  is 
also  the  case  in  Europe  ;  consequently  the  change  in  the  mammals 
seems  very  abrupt.  The  change  consists  partly  in  the  extinction 
of  old  types,  partly  in  the  immigration  of  new  forms,  and  partly  in 
the  development  of  the  native  stocks  to  more  advanced  grades. 
New  arrivals  are  the  Mastodons,  a  primitive  type  of  elephant,  and 
the  true  Ruminants.  The  earliest  American  forms  of  the  latter 
(Blastomeryx  and  Cosoryx)  have  curious  horns,  somewhat  like 


MIOCENE  517 

deer's  antlers.  A  number  of  weasel-  and  otter-like  Carnivores 
came  in  from  the  Old  World,  while  the  Wolves,  Panthers,  and 
Sabre-tooth  Tigers  were  very  numerous.  Besides  the  true  Rumi- 
nants, the  American  type  of  Camels  and  Llamas  continued  to 
flourish  in  such  genera  as  Procamelus,  PliaucJienia,  and  others. 
The  Loup  Fork  Horses  {Protohippus  and  Hippotherium)  are  much 
more  modern  in  character  and  larger  in  size  than  their  predeces- 
sors, but  still  have  three  toes  on  each  foot.  The  Rhinoceroses  are 
very  abundant,  and  form  a  peculiar  American  genus  (Aphelops}  of 
massive,  hornless  animals.  The  Atlantic  coast  Miocene  has  yielded 
numbers  of  Dolphins,  Sperm  and  Whalebone  Whales. 


FIG.  168.  —  Skeleton  of  Aphelops  fossiger.     (Osborn.) 

In  Europe  the  Upper  Miocene  mammals  were,  in  general,  like 
those  of  North  America,  but  a  salient  difference  is  in  the  much 
greater  number  of  early  types  of  Deer  and  Antelopes  which  are 
found  there,  together  with  various  forms  of  Swine  and  ancestral 
Bears.  Besides  the  Mastodons,  which  were  common  to  both  con- 
tinents, Europe  had  in  Dinotherium  a  remarkable  kind  of  elephant ; 
this  animal  had  a  much  flattened  head  and  a  pair  of  massive, 
backwardly  curved  tusks  in  the  lower  jaw. 

The  climate  of  the  early  Miocene  was  much  like  that  of  the  Oli- 
gocene  and  decidedly  warmer  in  Europe  than  in  North  America, 
though  it  was  mild  even  in  the  latter.  The  difference  seems  to  have 
been  largely  due  to  the  manner  in  which  Europe  was  intersected 
by  arms  and  gulfs  of  the  warm  southern  sea.  In  the  Upper  Miocene 
the  climate  became  somewhat  cooler  on  both  sides  of  the  ocean. 


518  THE  TERTIARY  PERIOD 

THE  PLIOCENE  EPOCH 

The  term  Pliocene  is  from  the  Greek  pleion,  more,  and  kainos, 
and  refers  to  its  close  approximation  to  the  present  order  of  things. 

American.  —  The  Pliocene  is  not  a  conspicuous  formation  in 
this  country,  and  only  of  late  years  has  it  been  recognized  at  all 
on  the  Atlantic  coast.  The  movements  which  closed  the  Miocene 
gave  to  the  Atlantic  and  Gulf  shores  nearly  their  present  outlines, 
but  some  differences  may  be  noted.  Much  of  southern  Florida 
was  still  under  water,  and  a  gulf  invaded  northern  Florida,  cover- 
ing a  narrow  strip  of  Georgia  and  South  Carolina.  Isolated  patches 
of  Pliocene  rocks  in  North  Carolina  and  Virginia  may  be  remnants 
of  a  continuous  band.  A  small  part  of  eastern  Mexico,  much  of 
Yucatan,  and  some  of  Central  America  were  still  submerged. 

On  the  Pacific  coast  the  post-Miocene  upheaval  had  laid  bare 
the  western  foot-hills  of  the  Sierra  and  greatly  disturbed  the  Mio- 
cene strata  of  the  Coast  Range.  This  range  sank  again  early  in 
the  Pliocene,  whose  strata  lie  unconformably  upon  the  Miocene, 
and  extend  over  upon  older  beds.  The  transgression  of  the  sea 
was  limited,  and  Pliocene  rocks  form  only  a  narrow  band  along 
the  coast  in  California,  Oregon,  and  Washington.  The  San  Fran- 
cisco peninsula  was  an  area  of  subsidence  and  maximum  deposi- 
tion, for  here  no  less  than  5800  feet  of  sandstone  (the  Merced 
series)  were  formed,  the  thickest  mass  of  Pliocene  in  North 
America.  The  mountains  of  British  Columbia  are  believed  to 
have  been  at  a  higher  level  than  now,  an  elevation  which  probably 
connected  Vancouver's  and  the  Queen  Charlotte  Islands  to  the 
mainland.  Marine  Pliocene  also  occurs  in  southern  Alaska. 

In  the  interior  are  a  number  of  Pliocene  lake  basins,  the  out- 
lines of  which  have  not  yet  been  determined.  The  oldest  of  these 
formations  is  the  Goodnight  stage,  named  from  a  locality  in  Texas, 
where  the  beds  lie  unconformably  upon  the  Nebraska  substage  of 
the  Loup  Fork.  Similar  beds  have  been  found  in  northern  Kansas 
and  eastern  Oregon,  where  they  are  closely  connected  with  the 
Nebraska  and  followed  them  after  no  long  interval.  The  second 
stage  of  fresh-water  Pliocene  is  the  Blanco,  which  extends  over 


PLIOCENE  519 

northwestern  Texas  into  Oklahoma.  Another  small  Pliocene  lake 
occurred  in  southern  Idaho,  but  its  place  in  the  series  is  not  yet 
known. 

The  volcanic  activity  in  the  Rocky  Mountain  and  Pacific  coast 
regions,  which  had  begun  in  the  Cretaceous,  continued  through 
the  Pliocene.  The  great  outflow  of  rhyolite  which  built  up  the 
Yellowstone  Park  plateau  is  referred  to  the  Pliocene.  Near  the 
end  of  the  epoch,  or  perhaps  after  its  close,  occurred  the  enor- 
mous fissure  eruptions,  which  flooded  northern  California  and 
Nevada,  southern  Idaho,  eastern  Oregon  and  Washington,  with 
thick  sheets  of  basalt,  obliterating  the  valleys  and  revolutionizing 
the  system  of  drainage. 

A  problematical  formation  is  the  Lafayette,  whose  geological 
position  and  mode  of  origin  are  still  debated.  The  Lafayette  is 
a  belt  of  sands  and  gravels  which  runs  through  Maryland,  Virginia, 
the  Carolinas,  and  the  Gulf  States,  around  the  southern  end  of  the 
Appalachians,  up  to  southern  Illinois,  whence  it  turns  southwest- 
ward  to  Texas.  As  in  the  typical  exposures  the  Lafayette  rests 
unconformably  upon  the  Miocene  and  is  unconformably  overlaid 
by  the  Pleistocene,  many  authorities  refer  it  to  the  Pliocene  and 
regard  it  as  a  marine  formation,  while  others  believe  it  to  be  Pleis- 
tocene and  to  have  been  largely  formed  by  waters  derived  from 
the  melting  of  the  first  ice-sheet.  The  almost  complete  absence 
of  fossils  is  a  great  obstacle  to  the  settling  of  these  questions. 

At  or  near  the  close  of  the  Pliocene,  extensive  upheavals  took 
place  in  several  different  parts  of  the  continent,  especially  on  the 
Pacific  slope.  The  rise  of  the  Rocky  Mountains  continued,  rais- 
ing the  western  part  of  the  Miocene  lake  beds  3000  feet  higher 
than  the  eastern.  The  height  of  the  Sierra  was  greatly  increased 
by  the  rise  of  the  mountains  along  the  eastern  fault  plane  and  the 
tilting  of  the  whole  block  westward.  The  new  valleys  cut  through 
the  late  basalt  sheets  of  the  Sierras  are  much  deeper  than  the 
older  valleys  excavated  in  Cretaceous  and  Tertiary  times,  which 
is  due  to  the  greater  height  of  the  mountains  and  consequent 
greater  fall  of  the  streams.  The  fault  blocks  which  form  the 
Basin  Ranges  were  still  further  displaced,  increasing  their  height. 


520  THE  TERTIARY   PERIOD 

The  Wasatch  Mountains  and  the  High  Plateaus  of  Utah  and  Ari- 
zona were  again  upraised.  The  great  mountain  range  of  the  St. 
Elias  Alps,  in  southeastern  Alaska,  was  upheaved  at  this  time,  or 
even  later,  and  the  mountains  of  British  Columbia  were  probably 
raised  still  higher.  In  Washington  and  Oregon  the  uplift  was 
small,  but  became  much  greater  in  southern  California,  reaching 
2500  feet  in  the  Monte  Diablo  range.  On  the  eastern  side  of  the 
continent  the  uplift  was  on  a  much  more  restricted  scale,  not 
generally  exceeding  100  feet.  The  Florida  anticline  underwent 
renewed  compression,  which  increased  its  height;  in  Georgia,  the 
continuation  of  this  fold  rose  to  400  feet.  The  same  movement 
extended  the  coast  of  Mexico  and  Central  America  and  brought 
the  continent  to  nearly  its  present  outlines. 

It  is  not  necessary  to  suppose  that  all  these  movements  were 
contemporaneous ;  merely  that  they  occurred,  now  in  one  place, 
now  in  another,  at  or  near  the  end  of  the  Pliocene  epoch. 

Foreign.  —  In  Europe  the  sea  generally  retreated  at  the  end  of 
the  Miocene,  leaving  in  the  north  only  Belgium  and  a  small  part  of 
northern  France  under  water.  In  England  the  sea  advanced  upon 
the  land ;  while  in  the  Mediterranean  region  large  areas  remained 
under  water,  as  in  Spain,  Algeria,  nearly  all  of  central  and  southern 
Italy  and  Sicily,  and  Greece.  In  this  region  volcanic  activity 
was  intense,  and  ^Etna,  Vesuvius,  and  the  volcanoes  of  central 
Italy  had  begun  their  operations.  Many  bodies  of  fresh  and 
brackish  water  existed  over  Europe,  an  older  stage  of  the  fresh- 
water Pliocene  occurring  in  southern  France,  near  Athens,  on  the 
Island  of  Samos,  and  in  Persia.  Over  the  region  of  the  great  Sar- 
matian  Sea  of  the  Upper  Miocene  were  numerous  bodies  of  brack- 
ish water,  in  which  lived  shells  much  like  those  which  now  inhabit 
the  Caspian.  In  India  was  a  lake  on  the  south  side  of  the  Him- 
alayas, the  deposits  of  which  now  make  the  Siwalik  Hills,  famous 
for  their  fossil  bones ;  and  similar  deposits  with  the  same  fossils 

occur  in  Borneo. 

Pliocene  Life 

The  life  of  the  Pliocene  is  very  modern  in  character.  Little  is 
known  of  the  vegetation  in  North  America,  but  in  Europe  it  is 


PLIOCENE 


521 


marked  by  the  continued  disappearance  of  the  characteristically 
tropical  plants  and  by  an  approximation  to  the  modern  European 
flora.  Many  trees  persisted,  however,  which  are  no  longer  native 
to  that  continent,  but  are  still  found  in  eastern  Asia  or  in  North 
America,  such  as  Tulip  Trees,  Magnolias,  Sequoias,  etc. 


PLATE  XII.    TERTIARY  FOSSILS  FROM  FLORIDA 

I.  Marginella  aurora,  3/4,  Miocene.  2.  Nassa  bidentata,  3/4,  Miocene  and 
Pliocene.  3.  Murex  Conradi,  2/3,  Miocene.  4.  Natica  floridana,  1/2,  Miocene. 
5.  Mitra  Wilcoxi,  1/2,  Miocene.  6.  Fasciolaria  tulipa,  1/2,  Pliocene.  7.  Typhis 
floridana,  Pliocene.  8.  Turbo  rectogrammicus,  1/2,  Pliocene.  (After  Dall.) 

Marine  Invertebrates  are  nearly  identical  with  modern  forms, 
and  the  great  majority  of  Pliocene  species  of  shells  are  still  living. 

The  Mammals  are  still  somewhat  behind  their  modern  succes- 
sors, though  much  more  advanced  than  their  predecessors.  Those 


522  THE  TERTIARY   PERIOD 

of  North  America  are  still  incompletely  known,  and  the  list  is  a 
short  one.  Mastodons,  Horses,  Rhinoceroses,  Peccaries,  and 
very  large  Llamas  represent  the  hoofed  animals.  Besides  the 
Dogs,  Cats,  and  Mustelines,  occur  flesh-eaters,  which  are  referred 
to  the  Hyaenas.  If  the  reference  is  correct,  this  is  the  only  occur- 
rence of  these  animals  in  America.  The  effects  of  the  connection 
with  South  America  are  seen  in  the  appearance  of  the  gigantic 
Sloths  and  Armadillos,  and  of  southern  families  of  Rodents. 

The  early  Pliocene  mammals  of  southern  Europe  closely  re- 
semble those  of  modern  Africa,  —  Wolves,  Cats,  Civets,  Hyaenas, 
Monkeys,  Rhinoceroses,  three-toed  Horses,  Deer  (of  which  Africa 
has  none) ,  a  great  variety  of  Antelopes  and  of  Giraffe-like  forms, 
and  Swine.  Mastodon  and  Dinotheriun  persisted,  the  latter  attain- 
ing great  size.  India  had  a  similar  fauna,  with  certain  geographi- 
cal differences.  Especially  to  be  noted  are  the  great  variety  of 
Oxen,  the  presence  of  Bears,  true  Elephants,  and  the  Hippopota- 
mus, of  the  first  Old  World  Camels,  and  of  the  extraordinary 
Sivatherium  and  Brahmatherium,  great,  four-horned  creatures 
allied  to  the  Giraffes.  In  the  Upper  Pliocene  the  Elephants,  Oxen, 
Hippopotamus,  and  Bears  had  extended  their  range  to  Europe, 
but  not,  so  far  as  we  know,  to  North  America. 

The  climate  of  the  Pliocene  was  evidently  cooler  than  that  of 
the  Miocene,  as  is  shown  by  the  changes  in  the  character  of  the 
vegetation  and  of  the  marine  shells.  The  inference  as  to  climatic 
change  may  be  made  with  unusual  confidence  in  this  case,  for 
nearly  all  the  Pliocene  species  of  shells  are  still  living,  and  can 
hardly  have  changed  their  habits.  In  the  English  Pliocene  the 
proportion  of  Arctic  shells  rises  from  5%  in  the  oldest  to  over 
60%  in  the  newest  beds.  The  refrigeration  was  greater  in  the 
sea  than  on  the  land,  for  the  vegetation  shows  that  the  air  had 
not  yet  grown  cold.  That  was  to  come  later. 

THE  SOUTH  AMERICAN  TERTIARY 

South  America  has  comparatively  little  marine  Tertiary.  A 
narrow  band  of  such  strata  is  found  on  the  Pacific  coast,  and 


SOUTH   AMERICAN  523 

some  occur  in  Patagonia  and  Argentina.  In  these  countries 
extensive  and  most  richly  fossiliferous  fresh-water  deposits  are 
found  interstratified  with  the  marine.  The  succession,  so  far  as 
known,  is  as  follows  :  — 

4.  Araucanian  Series,  or  Stage. 
3.  Santa-Cruzian  Series,  or  Stage. 
2.  Patagonian  Series,  or  Stage. 

(  Subpatagonian  Stage. 

\  Pyrotherium  Beds. 

The  Subpatagonian  and  most  of  the  Patagonian  are  marine,  the 
others  lacustrine.  The  mammals  which  are  found  more  abun- 
dantly than  in  any  other  known  deposits  are,  in  all  but  the  last 
of  these  formations,  totally  different  from  those  of  the  northern 
continents.  There  are  no  Artiodactyls,  Perissodactyls,  Elephants, 


FIG.  169.  — Skeleton  of  Toxodon  platense.    (Lydekker.) 

or  Mastodons,  neither  Condylarthra  nor  Amblypoda;  no  Car- 
nivores, Creodonts,  Insectivores,  or  Bats.  The  very  numerous 
genera  of  hoofed  animals  all  belong  to  orders  unknown  in  the 
north, —  Toxodontia  (Fig.  169),  Typotheria,  and  Litopterna. 
There  are  Monkeys  of  South  American  type,  and  great  numbers 
of  Rodents,  also  South  American,  belonging  exclusively  to  the 
Porcupine  series  (Hystricomorpha) ;  no  Squirrels,  Marmots,  Bea- 
vers, Mice,  or  Rabbits  occur  among  them.  A  marvellous  number 
of  Edentates,  Sloths,  Armadillos,  and  Ant-eaters  are  found ;  and, 
most  remarkable  of  all,  numerous  Australian  types  of  Marsupials, 
both  herbivorous  and  carnivorous,  are  a  characteristic  feature. 


524  THE  TERTIARY   PERIOD 

Isolation  from  North  America  and  some  sort  of  a  connection  with 
Australia  are  clearly  shown  by  these  mammalian  faunas.  In  the 
Araucanian  formation  the  northern  forms  at  last  make  their 
appearance.  Wolves,  Mastodons,  Horses,  Tapirs,  Deer,  Llamas, 
have  found  their  way  to  South  America,  just  as  southern  Sloths 
and  Armadillos  occur  in  the  northern  Pliocene. 

From  the  absence  of  means  forwiirect  comparison,  there  has 
been  much  dispute  concerning  the  *rue  position  of  these  South 
American  formations  in  the  Tertiary  system.  The  only  point  so 
far  at  all  well  established  is  that  the  Araucanian  beds  cannot  be 
older  than  the  Pliocene,  for  we  have  much  independent  geological 
evidence  to  prove  that  the  junction  of  North  and  South  America 
was  made  by  the  post-Miocene  movement. 


CHAPTER   XXXII 
THE  QUATERNARY  PERIOD  — (OR  PLEISTOCENE) 

IN  regions  to  which  the  great  ice-sheets  of  the  Glacial  epoch 
did  not  extend  the  transition  from  the  Tertiary  to  the  Quaternary 
is  perfectly  gradual,  so  that  it  is  often  difficult  or  impossible  to 
determine  to  which  division  a  given  set  of  strata  should  be  referred. 
The  seas  at  the  end  of  the  Pliocene  had  nearly  the  same  extension 
as  at  present,  and  on  the  same  coasts  the  same  deposits  continued 
to  form.  Even  the  Pliocene  coral  reefs  continued  uninterruptedly 
into  the  Pleistocene,  so  that  any  separation  at  all  seems  arbitrary. 
In  the  north,  however,  we  find  a  very  different  state  of  things. 
In  immense  regions  of  North  America,  Europe,  and  Asia  occur 
wide-spread  evidences  of  vast  glaciers  and  ice-sheets,  in  great 
masses  of  drift  which  cover  the  plains  and  choke  the  valleys,  in 
successive  lines  of  moraines,  in  great  erratic  blocks,  sometimes 
weighing  thousands  of  tons,  which  have  been  carried  long  distances 
from  their  parent  ledges,  and  in  the  scored  and  polished  rocks, 
the  rdches  moutonnees  and  other  features  which  we  have  learned 
to  be  characteristic  of  ice  action.  The  fossils  also  show  the 
prevalence  of  a  cold  climate  in  these  latitudes,  and  thus  all  the 
testimony  agrees  as  to  the  great  expansion  of  ice-sheets  and  gla- 
ciers in  the  early  Quaternary. 

THE  GLACIAL  OR  PLEISTOCENE  EPOCH 

At  the  end  of  the  Pliocene,  it  is  believed  that  the  North  Ameri- 
can continent  was  at  a  higher  level  than  now,  especially  to  the  north, 
which  favoured  the  accumulation  of  great  masses  of  snow.  It  is 
still  a  matter  of  debate  whether  there  was  a  single  Glacial  age  or 
epoch,  when  the  ice-sheet,  once  established,  had  rnany  episodes  of 

525 


526  THE  PLEISTOCENE   EPOCH 

advance  and  retreat,  yet  never  entirely  disappeared,  or  whether 
there  were  several  Glacial  and  Interglacial  ages,  when  the  ice  alter- 
nately advanced  and  was  completely  melted  away.  There  is  much 
to  be  said  on  both  sides  of  this  question,  but  the  present  tendency 
among  students  of  the  Glacial  epoch  is  to  favour  several  distinct 
Glacial  and  Interglacial  ages,  one  of  the  strongest  arguments  for 
which  is  the  evidence  given  by  the  fossils  of  the  return  of  mild 
and  even  warm  climates.  Professor  James  JGeikie  accepts  no 
less  than  six  Glacial  and  five  Interglacial  stages  for  Europe,  and 
Professor  Chamberlin  finds  evidence  for  five  Glacial  and  four  Inter- 
glacial stages  in  North  America. 

American.  —  At  the  times  of  great  expansion  the  ice-sheets 
covered  nearly  all  of  North  America  down  to  latitude  40°  N.,  an- 
ticipating the  conditions  of  modern  Greenland,  though  on  a  vastly 
larger  scale.  Three  distinct  centres  or  areas  of  maximum  accumu- 
lation of  the  ice  have  been  identified  in  northern  Canada,  from 
which  the  great  ice-sheets  flowed  outward  in  all  directions,  though 
each  one  of  the  sheets  had  its  own  episodes  of  advance  and  retreat, 
so  that  the  same  region  of  country  was  overflowed,  now  by  exten- 
sions from  one  sheet,  and  again  by  those  from  another.  One  of 
these  centres  of  accumulation  and  distribution  lay  to  the  north  of 
the  St.  Lawrence  River,  and  on  the  highlands  of  Labrador,  send- 
ing its  ice-mantle  southward  over  the  Maritime  Provinces,  New 
England,  and  the  Middle  States,  as  far  west  as  the  Mississippi 
River.  This  is"  called  the  Laitrentide  Ice-sheet  or  Glacier.  A 
second  centre  was  near  the  west  coast  of  Hudson  Bay,  and  from 
this  area  the  ice  streamed  outward  in  all  directions  westward 
toward  the  Rocky  Mountains,  northward  to  the  Arctic  Ocean, 
eastward  into  Hudson  Bay,  southward  through  Manitoba  into  the 
Dakotas,  Minnesota,  and  Iowa.  This  great  ice-sheet  has  been 
named  the  Keewatin  Glacier,  from  the  Canadian  district  of  that 
name.  A  third  centre  was  formed  by  the  Cordillera  of  British 
Columbia,  which  for  a  distance  of  1200  miles  was  buried  under 
a  great  ice-mantle  that  flowed  both  to  the  northwestward  and 
southeastward.  The  thickness  of  the  ice  in  these  vast  flows  was 
very  great ;  over  New  England,  the  scorings  on  the  mountain  sides 


DEPOSITS   BY   ICE-SHEETS  527 

show  that  the  ice  was  several  thousand  feet  deep,  only  the  highest 
peaks  rising  through  it  as  nunataks. 

In  addition  to  the  great  ice-sheets  which  covered  the  northern 
parts  of  the  continent,  large  local  glaciers  were  developed  in  the 
Rocky  Mountains,  the  Sierra  Nevada,  and  other  ranges  of  the 
western  Cordillera.  In  these  mountains  almost  every  valley  shows 
the  evidences  of  former  glaciation,  both  in  its  scored  and  polished 
sides  and  bed,  and  in  the  lines  of  moraine  at  its  opening.  Alaska, 
strange  to  say,  was  not  glaciated,  except  locally,  none  of  the  great 
ice-sheets  extending  to  it.  Regarding  the  Great  Plains  region 
there  is  some  difference  of  interpretation  ;  by  some  authorities, 
especially  Dr.  G.  M.  Dawson,  it  is  believed  that  the  northern  plains 
were  covered  with  water  and  floating  ice,  while  others  suppose  that 
the  Keewatin  and  Cordilleran  ice-sheets  were  joined,  and  buried 
all  the  country  through  northern  Montana,  Idaho,  and  Washington 
to  the  Pacific. 

Deposits  by  the  Ice-Sheets.  — The  general  name  for  the  materials 
deposited  by  the  vast  ice-sheets  and  the  waters  derived  from  them 
is  Drift,  which  is  both  stratified  and  unstratified.  The  unstratified 
drift  is  that  which  is  made  by  the  action  of  the  ice  alone,  and 
assumes  several  forms,  (i)  Moraines,  lateral  and  terminal.  These 
have  already  been  described  and  need  not  detain  us  further. 
(2)  Till  is,  when  typically  developed,  a  sheet  of  tough  clay,  which 
may  be  more  or  less  sandy,'  crowded  with  stones  and  boulders  of 
various  sizes,  and  with  no  regularity  of  arrangement.  Most  of 
the  stones  and  boulders  show  the  evident  marks  of  ice  action, 
both  in  their  form  and  in  the  scoring  and  polishing  to  which  they 
have  been  subjected.  The  materials  of  till  are  principally  derived 
from  the  bed  rock  upon  which  it  rests,  or  from  some  spot  close 
at  hand,  most  of  these  materials  having  been  transported  only  for 
short  distances.  A  certain  proportion  of  the  stones,  however,  have 
been  carried  for  long  distances,  as  may  be  shown  by  tracing  them 
to  the  ledges  whence  they  were  derived.  Till  is  supposed  to  be 
the  ground  moraine  of  the  ice- sheet,  left  behind  and  overridden 
when  the  descending  ice  reached  more  level  ground,  and  packed 
into  depressions.  This  explanation  is,  to  some  extent,  conjectural, 


528  THE   PLEISTOCENE  EPOCH 

because  the  formation  of  similar  deposits  has  not  been  observed 
in  connection  with  modern  glaciers ;  and  several  geologists  do  not 
accept  the  commonly  received  view  of  the  origin  of  till,  but  regard 
it  as  the  product  of  water  and  floating  ice. 

Stratified  Drift  is  made  by  water,  either  alone  or  assisted  by 
the  action  of  ice.  Much  of  the  great  mantle  of  Pleistocene  drift 
is  more  or  less  completely  stratified,  because  the  border  of  the 
ice-sheets,  whether  they  were  advancing  or  retreating,  was  melting, 
and  the  drift  left  by  the  melting  ice  in  its  retreat  was  worked 
over  more  or  less  by  water.  Subglacial  streams  are  active  agents  of 
deposition,  both  while  still  under  the  ice  and  after  they  have 
emerged.  In  long,  winding  tunnels  beneath  the  ice  they  may 
leave  the  gravel  ridges  called  eskers.  Subglacial  streams  often  are 
under  great  pressure  and  rise  like  fountains  from  beneath  the  ice- 
edge  ;  the  relief  of  pressure  on  escaping  causes  deposition  at  the 
edge  of  the  ice-sheet.  Drift  thus  piled  in  irregularities  of  the 
ice-front  formed  kames,  which  are  hillocks  or  short  ridges  of  strati- 
fled  drift,  shaped  by  the  recesses  of  the  ice  in  which  they  gathered, 
and  are  often  connected  with  the  terminal  moraines.  Beyond  the 
free  border  of  the  ice  the  escaping  waters  spread  out  stratified  drift 
for  considerable  distances.  When  these  waters  descended  valleys 
that  were  not  too  steep,  they  deposited  sand  and  gravel  in  their 
descent,  forming  valley  trains,  which  are  of  coarser  materials  and 
steeper  grade  near  their  heads  than  below.  When  the  waters 
escaped  from  the  ice  away  from  valleys  they  spread  out  their 
deposits  as  morainic  or  overwash  plains.  The  ice-front  in  several 
places  entered  the  sea,  and  in  others  formed  lakes  by  damming 
valleys  and  depressions ;  the  ice-derived  materials  were,  in  such 
cases,  rapidly  deposited  in  quiet  waters,  as  deltas  and  subaqueous 
overwash  plains,  and  the  finer  silt  and  clay  were  carried  further 
out  into  deeper  water. 

When  the  ice-cap  was  retreating,  the  processes  described  were 
continued  along  the  shrinking  border,  until  every  part  of  the  area 
once  covered  by  the  ice  had  been  subjected  to  them,  and  thus 
the  moraines  left  by  the  ice  were  covered  with  a  more  or  less 
extensive  and  thick  mantle  of  the  stratified  drift. 


ALBERT  AN   STAGE  539 

Along  the  border  of  an  advancing  ice-sheet,  the  same  phe- 
nomena were  repeated,  but  the  kames,  valley  trains,  lake  deposits, 
etc.,  were  incomplete  because  of  the  continually  advancing  ice, 
which  overrode  them,  and,  it  may  be,  ploughed  them  up,  or  buried 
them  under  ground  moraines,  or  otherwise  modified  them.  The 
surface  deposits  of  a  glaciated  region  are  those  made  by  the  ice 
and  water  during  the  final  retreat. 

Evidently,  a  succession  of  glacial  episodes  of  alternately  en- 
croaching and  shrinking  ice-sheets  must  produce  an  exceedingly 
complex  succession  of  stratified  and  unstratified  drift,  and  it  can 
cause  little  surprise  that  interpretations  of  such  obscure  phenom- 
ena should  differ  widely.  If  the  successive  deposits  were  sepa- 
rated by  long,  truly  interglacial  periods,  then  the  sheets  of  drift 
must  have  been  exposed  to  the  denuding  agencies  for  correspond- 
ing lengths  of  time,  and  will  exhibit  the  various  stages  of  chemical 
and  mechanical  disintegration  appropriate  to  the  length  of  expos- 
ure. There  should  be  a  manifest  difference  in  this  respect,  be- 
tween the  earlier  and  the  later  deposits  of  drift. 

For  the  reasons  indicated,  the  chronological  arrangement  of 
the  various  parts  of  the  drift,  and  the  correlation  of  the  deposits, 
glacial,  lacustrine,  and  marine,  of  different  regions  of  the  conti- 
nent are  exceedingly  difficult.  The  following  classification  has  re- 
cently been  proposed  by  Chamberlin  for  the  drift  of  the  Mississippi 
valley,  but  is  only  tentative  and  provisional. 

9.   Wisconsin  Till-sheets  (earlier  and  later). 

8.    Interglacial  deposits  (?  Toronto). 
7.    lowan  Till-sheet. 


Glacial  or 
Pleistocene  Series. 


6.    Interglacial  deposits. 
5.    Illinois  Till-sheet. 

4.    Interglacial  deposits  (Buchanan). 
3.    Kansan  Till-sheet. 

2.    Interglacial  deposits  (Aftonian). 
I.   Albertan  Drift-sheet. 

The  Albertan  stage  is  typically  displayed  in  the  Canadian  prov- 
ince of  Alberta,  where  the  first  formation  of  drift  was  due  to  the 
extension  of  glaciers  eastward  from  the  Rocky  Mountains.  Far- 


530  THE   PLEISTOCENE   EPOCH 

ther  east  the  till-sheet  passes  into  the  Saskatchewan  gravels  laid 
down  by  the  waters  derived  from  the  ice-front.  The  materials  of 
this  drift  are  either  local  or  came  from  the  Rocky  Mountains. 
The  lowest  drift  deposits  of  the  Mississippi  valley  are  provisionally 
regarded  as  equivalents  of  the  Albertan  drift ;  they  are  best  dis- 
played in  southern  Iowa,  but  their  extent  has  not  yet  been  deter- 
mined. According  to  some  authorities  the  Lafayette  formation 
should  be  correlated  with  the  earliest  glacial  stage,  rather  than 
with  the  Pliocene. 

A  great  retreat  of  the  ice,  if  not  its  entire  disappearance,  brought 
about  interglacial  conditions  at  least  in  the  Mississippi  valley 
(Aftonian  stage).  The  surface  exposed  by  the  retiring  ice  was 
rapidly  occupied  by  vegetation,  which  in  many  places  in  Iowa, 
Minnesota,  etc.,  formed  accumulations  of  peat,  sometimes  to  the 
depth  of  25  feet.  The  Kansan  stage  represents  the  greatest 
extension  southwestward  of  the  ice-sheet,  when  the  glacier  de- 
scended from  the  north  (perhaps  the  Keewatin  glacier)  nearly  to 
the  mouth  of  the  Ohio  River,  and  spread  across  Iowa  and  Mis- 
souri far  into  Kansas.  Eastward,  the  sheet  extended  across  the 
Mississippi  River  into  Illinois.  Again  came  a  time  of  retreat,  when 
the  Kansan  till  was  eroded,  soil  was  formed,  and  peat  deposited 
upon  it  {Buchanan  stage) .  A  renewed  extension  of  the  ice  laid 
down  the  Illinois  till-sheet,  which  is  found  not  only  in  that  state, 
but  crossed  into  Iowa  also.  A  fourth  recrudescence  of  the  glacier 
(lowan  stage)  occasioned  the  deposit  of  another  tilt-sheet,  of  an 
extent  not  yet  determined,  which  is  best  displayed  in  northeastern 
Iowa,  where  it  is  intimately  associated  with  the  largest  accumula- 
tions of  loess  in  the  Mississippi  valley.  The  lowan  till-sheet  is 
followed  by  interglacial  deposits  which  are  perhaps  contempora- 
neous with  those  which  are  so  well  shown  near  Toronto  on  Lake 
Ontario.  The  latter  beds  form  a  succession  of  fine  shales  and 
sandstones  that  lie  between  two  sheets  of  glacial  drift,  and  con- 
tain fossil  plants  which  indicate  a  milder  climate  than  that  of 
the  present  time  at  Toronto,  near  which  these  beds  are  found. 
Such  a  fact  is  difficult  to  explain,  except  as  the  result  of  truly 
interglacial  conditions.  The  Wisconsin  stage  is  much  the  most 


LAKE   AGASSIZ  531 

conspicuous  and  best  known  of  all,  and  its  sheets  of  till  and  drift 
are  far  thicker  than  those  of  the  other  Glacial  stages.  Especially 
conspicuous  is  the  great  terminal  moraine  which  has  been  traced 
almost  across  the  continent.  Beginning  at  Nantucket,  the  moraine 
runs  through  Long  Island  and  Staten  Island  to  New  Jersey,  which 
it  crosses  into  Pennsylvania ;  here  it  bends  sharply  to  the  north- 
west to  the  boundary  of  New  York,  but  turns  southwest  almost 
at  a  right  angle,  reaching  nearly  to  the  Ohio  River  at  Cincinnati. 
It  crosses  in  an  irregular,  sinuous  line  the  states  of  Indiana,  Illinois, 
Iowa,  and  thence  northwestward  through  the  Dakotas  and  Montana 
into  Canada,  where  it  probably  reaches  the  Arctic  Sea. 

In  New  England  there  is  no  clear  evidence  of  more  than  one 
Glacial  stage.  In  part,  this  may  be  due  to  the  later  development 
of  the  Laurentide  glacier.  The  geologists  of  the  Canadian  Survey 
believe  that,  "  beginning  at  the  west  and  going  eastward,  these 
three  great  glaciers  [i.e.  the  Cordilleran,  Keevvatin,  and  Laurentide] 
reached  their  widest  extent  and  retired  in  succession."  (Tyrrell.) 

The  final  retreat  of  the  ice  was  by  slow  stages  with  many  halts. 
In  the  central  West  are  preserved  many  lines  of  moraine,  with 
kettle-holes,  kames,  and  drumlins,  which  mark  the  successive 
pauses  in  the  retreat. 

An  interesting  episode  of  later  Glacial  times  was  the  formation 
in  Minnesota  and  Manitoba  of  a  great  body  of  fresh  water,  Lake 
Agassiz,  which  was  700  miles  long  from  north  to  south. 

The  great -  Keewatin  glacier,  which  had  long  occupied  the  basin 
of  Lake  Winnipeg  and  the  Red  River  valley,  began  eventually  to 
retreat  northward,  while  the  Laurentide  glacier,  which,  it  would 
seem,  had  begun  to  accumulate  at  a  somewhat  later  date,  gradually 
advanced  to  the  westward.  When  the  two  sheets  united,  the 
withdrawal  of  the  Keewatin  glacier  had  left  nearly  or  quite  all  of 
Manitoba  free  from  ice.  The  junction  of  the  Laurentide  and 
Keewatin  glaciers  formed  a  continuous  ice-wall  on  the  north  and 
east,  shutting  off  the  drainage,  which  seems  before  to  have  had  a 
free  course  to  Hudson  Bay,  and  damming  back  the  waters  into 
a  great  lake.  The  lake  rose  until  it  overflowed  southward  into 
the  Mississippi  by  means  of  the  now  extinct  Warren  River.  As 


532  THE   PLEISTOCENE   EPOCH 

this  river  excavated  its  bed,  it  gradually  lowered  the  water  level 
of  Lake  Agassiz.  The  drainage  of  the  lake  seems  to  have  been 
accomplished  by  the  continued  retreat  of  the  Keevvatin  glacier 
until  it  became  separated  from  the  Laurejitide  sheet,  leaving  the 
way  open  around  the  northern  edge  of  the  latter  to  Hudson  Bay. 

In  the  non-glaciated  parts  of  the  continent  occur  stratified 
Pleistocene  deposits,  which  it  is  very  difficult  to  associate  with  the 
events  taking  place  in  the  glaciated  area,  for  lack  of  any  means 
of  direct  comparison.  On  the  Atlantic  slope  from  New  Jersey 
southward  a  succession  of  Pleistocene  gravels  and  sands  constitutes 
the  Columbian  formation,  so  called  because  of  its  typical  develop- 
ment in  the  District  of  Columbia.  These  deposits  are  estuarine 
and  marine  deltas  and  shore  sediments,  partly  submarine.  Three 
phases  of  the  Columbian  have  been  recognized,  the  fluvial,  inter- 
fluvial,  and  low-level.  "  The  fluvial  phase  is  found  in  its  fullest 
development  along  the  leading  waterways.  It  consists  of  a  lower 
horizon  of  coarse  materials,  pebbles,  and  boulders,  generally  pass- 
ing upward  into  a  brownish  loam.  The  interfluvial  phase  is  found 
typically  represented  in  the  country  which  lies  between  the  water- 
ways, and  is  characterized  by  materials  of  local  origin  and  pro- 
duced largely  by  wave  action.  The  low-level  phase  is  developed 
throughout  the  area  farthest  removed  from  the  ancient  shore-line. 
The  deposits  consist  of  sands,  clays,  and  loams.  They  indicate 
a  much  less  disturbed  type  of  sedimentation  than  that  found  in 
either  the  fluvial  or  interfluvial  phase.  They  were  scattered  as 
a  coating  of  greater  or  less  thickness  over  the  eastern  portion 
of  the  district,  and  have  since  suffered  but  little  from  denudation. 
The  fossils  are  of  recent  species  and  indicate  the  marine  origin  of 
the  deposits."  (Clark.)  At  present  these  deposits  are  from  100 
to  400  feet  above  sea-level  and  indicate  a  submergence  and 
reelevation  of  the  middle  Atlantic  coast. 

Over  the  Great  Plains  from  South  Dakota  to  Texas  the  surface 
formation  is  a  fine,  calcareous,  sandy  clay,  which  lies  unconform- 
ably  upon  the  eroded  surfaces  of  older  strata,  from  the  Blanco  to 
the  Cretaceous.  This  formation  may  be  called  the  Sheridan  stage 
{Equus  Beds),  from  Sheridan  County,  Nebraska,  where  it  is 


CHAMPLAIN   STAGE  533 

admirably  shown,  and  is  Pleistocene  in  age,  probably  corresponding 
to  one  of  the  Glacial  stages,  though  by  some  it  is  regarded  as 
Pliocene. 

In  the  Great  Basin  the  Pleistocene  was  a  time  of  far  less  arid 
climate  than  at  present.  In  the  eastern  part  of  the  Basin  was 
established  the  great  Lake  Bonneville  (see  p.  146),  which  had  an 
outlet  to  the  north  into  the  Snake  River.  The  deposits  of  this 
lake  show  that  it  had  two  periods  of  expansion,  separated  by  one 
of  almost  complete  desiccation.  Lake  Lahontan,  on  the  western 
side  of  the  Great  Basin,  which  had  no  outlet,  had  similar  episodes 
of  rise  and  fall.  The  two  relatively  moist  periods  when  these 
lakes  were  high  may  correspond  in  time  to  the  two  stages  of 
greatest  advance  of  the  ice-sheets,  the  Kansan  and  Wisconsin,  but 
this  is  only  conjectural. 

In  the  Glacial  epoch  a  subsidence  had  begun  which  continued 
until  it  became  a  very  marked  feature  of  the  times.  The  depres- 
sion was  greatest  toward  the  north  and  especially  in  the  valley  of 
the  St.  Lawrence  ;  at  the  mouth  of  the  Hudson,  for  example,  the 
land  stood  about  70  feet  below  its  present  level,  on  the  coast  of 
Maine  150  to  300  feet,  and  in  the  St.  Lawrence  valley  500  to  600 
feet  below.  The  consequence  of  the  depression  was  that  an  arm 
of  the  sea  extended  up  the  St.  Lawrence  nearly  to  Lake  Ontario, 
which  was  little,  if  at  all,  above  sea-level.  Two  long  and  narrow 
gulfs  reached  out  from  this  sea,  one  up  the  valley  of  the  Ottawa 
River  and  the  other  over  Lake  Champlain.  The  lines  of  raised 
beaches,  the  sands  and  gravels  filled  with  marine  shells,  and  the 
bones  of  whales  and  walruses,  are  the  present  evidences  of  this 
submergence  (the  Champlain  stage).  These  beds,  so  named 
from  their  typical  exposure  of  the  shores  of  Lake  Champlain,  were 
formed  probably  after  the  St.  Lawrence  valley  had  been  freed 
from  the  ice-sheet,  but  it  is  uncertain  whether  they  were  contem- 
poraneous with,  earlier,  or  later  than  the  Columbian  formation. 

On  the  Pacific  coast  also  we  find  evidences  of  submergence. 
The  Chaix  Hills  in  Alaska  are  made  up  of  stratified  moraine  ma- 
terial 4000  to  5000  feet  thick  (see  Fig.  61,  p.  157),  and  at  cor- 
responding levels  Champlain  species  of  marine  shells  are  found. 


534  THE   PLEISTOCENE  EPOCH 

In  California  raised  beaches  occur  as  high  as  1500  feet  above  the 
present  sea-level,  which  were  probably  due  to  the  same  submer- 
gence, though  they  may  be  older.  Pleistocene  movements  con- 
tinued, it  may  be,  into  the  Recent  epoch,  increased  the  height 
of  the  Sierra  Nevada,  Wasatch,  and  Basin  Ranges,  and  of  the 
high  Plateaus  of  Utah  and  Arizona. 

The  late  Pleistocene  was  a  time  of  ameliorated  climate  and 
heavy  rainfall,  when  the  flooded  rivers  moved  sluggishly,  owing  to 
the  diminished  slope,  and  spread  sheets  of  sands,  gravels,  and 
clays  over  their  flood  plains  and  in  their  estuaries,  through  which 
they  have  subsequently  cut  terraces,  when  elevation  had  given 
them  renewed  power. 

The  events  of  the  Glacial  epoch,  and  the  diastrophic  movements 
which  accompanied  and  followed  it,  have  had  the  most  important 
and  wide-spread  effects  upon  the  topography  of  the  glaciated 
regions.  The  sheets  of  drift,  stratified  and  unstratified,  have  com- 
pletely changed  the  surface  of  the  country,  and  by  filling  up  the 
pre-Glacial  valleys,  have  revolutionized  the  drainage,  only  the 
largest  streams  being  able  to  regain  their  old  courses.  Innumer- 
able lakes,  large  and  small,  were  formed  in  depressions,  rock  basins, 
and  behind  morainic  dams,  the  contrast  between  the  glaciated  and 
non-glaciated  regions  in  regard  to  the  number  of  lakes  in  each 
being  very  striking.  The  events  which  led  up  to  the  formation 
of  the  great  Laurentian  lakes  are  connected  with  the  successive 
phases  of  the  Glacial  epoch,  but  they  are  very  complex  and  the 
phenomena  are  differently  interpreted  by  different  observers,  so 
that  only  the  briefest  outline  of  them  can  be  attempted  in  this 
place. 

When  the  Laurentide  glacier  was  retreating  across  the  area  now 
occupied  by  or  tributary  to  the  Great  Lakes,  its  front  acted  as  a 
great  dam  holding  back  the  waters  from  descending  the  valley 
of  the  St.  Lawrence,  and  forming  ice-dammed  lakes,  whose  form 
and  size  varied  with  the  position  of  the  ice-front.  These  lakes 
discharged  their  waters  southward  at  various  points  where  the 
divides  were  lowest.  At  one  time,  for  example,  when  the  eastern 
half  of  Lake  Ontario  was  filled  by  the  ice,  a  long  lake  extended 


FOREIGN  535 

along  the  ice-front  from  western  New  York  to  the  site  of  Chicago, 
where  was  the  outlet  to  the  Mississippi.  This  body  of  water 
occupied  parts  of  the  basins  of  the  present  Lakes  Ontario,  Erie, 
Huron,  and  Michigan.  When  the  continued  retreat  of  the  ice 
had  freed  the  basin  of  Lake  Ontario,  the  outlet  is  believed  to  have 
shifted  to  a  low  point  near  Rome  (N.Y.)and  to  have  discharged 
into  the  Mohawk  River,  a  change  which  lowered  the  water-level 
by  several  hundred  feet.  The  shallowed  waters  were  no  longer 
sufficient  to  cover  the  divides  between  the  distinct  lake  basins,  and 
three  lakes  were  formed,  —  Lake  Huron,  still  partly  walled  by  ice, 
Lake  Erie,  and  Lake  Iroquois,  which  occupied  the  Ontario  basin, 
but  was  larger  and  emptied  into  the  Mohawk,  for  the  St.  Lawrence 
channel  was  still  blocked  by  the  ice-front. 

None  of  these  lakes  had  the  size  and  position  of  their  modern 
representatives.  A  gentle  upheaval  of  the  region,  rising  toward 
the  northeast,  shifted  all  the  lakes  to  the  southwest.  Lake  Huron, 
which  had  previously  discharged  its  waters  (it  is  believed  by  some 
authorities)  by  way  of  the  Ottawa  River,  thus  was  given  an  outlet 
through  the  St.  Clair  River  and  became  joined  to  Lake  Erie, 
which  thus  received  the  overflow  from  the  upper  lakes.  The 
complete  removal  of  the  ice,  together  with  the  diastrophic  move- 
ments, gave  to  Lake  Ontario  its  present  size  and  shape  and  opened 
to  it  an  outlet  into  the  St.  Lawrence. 

The  Pleistocene  was  closed  and  the  Recent  epoch  inaugurated 
by  a  movement  of  upheaval  which  raised  the  continent  to  its 
present  height. 

FOREIGN  PLEISTOCENE 

The  Glacial  epoch  in  Europe  ran  a  course  remarkably  parallel 
with  its  history  in  North  America.  After  the  first  Glacial  and 
Interglacial  stages  (perhaps  representing  the  Albertan  and  Afto- 
nian),  came  the  time  of  the  greatest  expansion  of  the  ice,  the 
Saxonian  stage  of  Geikie,  which  is  believed  to  correspond  to  the 
Kansan  of  America.  The  great  centre  of  dispersion  was  the  Scan- 
dinavian peninsula,  where  the  ice  was  probably  6000  to  7000  feet 
thick,  and  whence  it  flowed  outward,  filling  the  Baltic  and  North 


536  THE   PLEISTOCENE   EPOCH 

Seas,  and  covering  Finland,  northwestern  Russia,  the  lowlands  of 
Germany,  and  extending  to  England.  The  Highlands  of  Scotland 
were  a  secondary  centre,  its  ice-sheets  flowing  into  the  North  Sea 
and  uniting  with  those  from  Scandinavia,  and  westward  to  the 
ocean.  The  Irish  Channel  was  also  filled  up.  From  the  south- 
west of  Ireland  to  the  North  Cape  of  Norway,  a  distance  of  2000 
miles,  was  probably  a  continuous  wall  of  ice  fronting  the  sea,  like 
that  which  now  surrounds  the  Antarctic  continent.  At  the  same  time 
the  Alps  were  the  seat  of  enormous  glaciers,  only  the  highest  peaks 
rising  above  the  sheets  of  ice,  and  these  great  glaciers  extended 
far  out  from  the  foot  of  the  mountains,  covering  all  the  lowlands 
of  Switzerland  and  extending  from  Austria  and  Bavaria,  on  the 
east,  to  the  Rhone  valley  near  Lyons,  on  the  west.  The  high 
plateau  of  Asia,  from  the  Himalayas  to  Bering's  Sea,  shows  evi- 
dences of  glaciation,  and  great  valley  glaciers  were  formed  on  the 
southern  slopes  of  the  Himalayas,  extending  in  some  places  to 
within  2000  feet  of  the  sea-level. 

A  second  great  Glacial  stage  (the  fourth  Glacial  or  Mecklen- 
burgian  of  James  Geikie)  is  generally  recognized  in  Europe  and 
correlated  with  the  Wisconsin  stage  of  America.  This  ice-sheet 
was  much  less  extensive  than  the  former  one,  being  confined  prin- 
cipally to  Finland,  Scandinavia,  the  Baltic  Sea,  which  it  filled, 
Denmark,  and  a  little  of  north  Germany.  The  prevailing  motion 
of  this  sheet  was  from  east  to  west.  The  Alpine  glaciers  were 
also  extended  far  beyond  their  present  limits,  but  not  so  widely  as 
before. 

Following  the  Mecklenburgian  stage  came  alternating  periods 
of  milder  and  colder  climates,  the  fourth  and  fifth  Interglacial, 
and  fifth  and  sixth  Glacial  stages  of  Geikie,  the  Glacial  stages 
marked,  not  by  the  formation  of  great  continental  ice-sheets,  but 
by  the  extension  or  recrudescence  of  local  snow-fields  and  valley 
glaciers.  Oscillations  of  level  also  occurred  along  the  coasts, 
allowing  limited  transgressions  of  the  sea. 

No  evidence  of  continental  ice-sheets  has  been  found  in  the 
Southern  Hemisphere,  but  high  mountains  like  the  Andes,  the 
New  Zealand  Alps,  and  the  Australian  Alps  had  very  large  glaciers, 


PLANTS  537 

probably  contemporaneous  with  those  of  the  Northern  Hemisphere, 
and  Patagonia  was  extensively  glaciated. 

The  causes  of  the  climatic  changes  which  led  up  to  the  Glacial 
epoch  and  to  the  later  disappearance  of  the  ice-sheets,  are  still 
wrapped  in  mystery.  Many  attempts  have  been  made  to  solve 
this  most  difficult  problem,  but  none  is  convincing  or  satisfactory. 

Pleistocene  Life 

The  frequent  and  extreme  climatic  changes,  of  which  we  find 
such  abundant  evidence  in  the  Pleistocene,  caused  extensive 
migrations  and  dispersions  of  animals  and  plants,  and  the  rapid 
succession  of  Arctic  and  temperate  forms.  Many  land  bridges 
between  different  continents,  or  between  continents  and  what  are 
now  islands,  were  not  severed  until  the  end  of  the  Pleistocene,  per- 
mitting migrations  which  are  no  longer  possible.  The  extension 
of  the  ice-sheets  brought  with  them  Arctic  floras  and  faunas,  which 
retreated  in  the  Interglacial  times,  while  temperate  animals  and 
plants  spread  northward  to  replace  them.  These  conditions  pro- 
duced a  very  severe  struggle  for  existence  and  were  fatal  to  a 
great  many  large  mammals,  causing  numerous  extinctions  over  the 
larger  part  of  the  world. 

Pleistocene  plants  are  almost  all  of  the  same  species  as  those 
now  living,  but  they  are  often  very  differently  distributed.  The 
Glacial  cold  greatly  impoverished  the  European  forests,  which  in 
the  Pliocene  had  many  kinds  of  trees  now  found  only  in  North 
America  or  in  eastern  Asia.  Owing  to  the  east  and  west  trend  of 
the  European  mountains,  the  forests  could  not  retire  before  the  ice, 
and  return,  as  they  did  in  the  United  States,  where  no  mountain 
barriers  shut  them  off  from  the  warm  latitudes  of  the  south. 
When  the  ice-sheets  melted  and  the  climate  was  ameliorated,  the 
Arctic  flora  and  fauna  were  forced  to  retreat  in  their  turn ;  they 
did  so  not  only  by  following  the  retiring  ice-front,  but  also  by 
ascending  the  mountains  as  the  latter  were  cleared  of  ice.  Thus, 
high  mountains  in  the  Northern  Hemisphere  have  on  their  upper 
slopes  an  Arctic  flora  and  fauna,  separated,  perhaps,  by  hundreds 
of  miles  from  the  nearest  similar  colony.  For  example,  the  higher 


538  THE   PLEISTOCENE   EPOCH 

parts  of  the  White  Mountains  have  plants  which  do  not  occur 
on  the  lowlands  until  Labrador  is  reached,  and  the  snowy  Alps 
have  truly  Arctic  plants  and  animals. 

Of  Pleistocene  animals  it  is  only  the  mammals  that  require 
mention.  Here  also  we  find  the  same  mingling  of  northern  and 
southern  forms,  and  association  of  types  now  widely  separated. 
North  America  had  Mastodons,  Elephants,  Horses,  Tapirs,  the 
first  Bisons  (which  had  migrated  from  the  Old  World,  as  had 
several  kinds  of  Deer  and  the  Musk-ox),  Peccaries  and  huge 
Llamas,  Wolves,  great  Cats  as  large  as  lions,  Sabre-tooth  Tigers, 
and  the  first  Bears,  also  immigrants.  A  great  variety  of  Rodents 
is  found,  most  of  them  kinds  which  still  inhabit  the  country,  but 
mingled  with  these  are  South  American  forms  like  the  Cavies  and 
Water  Hog  (ffydrothcerus) ,  and  the  giant  Beaver  (Castoroides] 
is  an  altogether  peculiar  form.  Enormous  Sloths  (Megatherium^ 
Mylodon,  and  Megalonyx)  and  Armadillos  ( Glyptodon)  show  that 
the  way  of  migration  from  the  south  was  still  open. 

In  South  America  were  an  astonishing  number  of  huge  Eden- 
tates :  Sloths  nearly  as  large  as  elephants,  Ant-eaters,  and  a  marvel- 
lous variety  of  giant  Armadillos.  Some  of  the  immigrants  from 
the  north,  which  are  now  extinct,  still  lingered  in  the  Pleistocene, 
such  as  the  Mastodons  and  Horses. 

Europe  was  the  meeting-ground  of  mammalian  types  now 
widely  scattered.  Together  with  Arctic  forms  like  the  Reindeer, 
Musk-ox,  Mammoth  (Hairy  Elephant),  Hairy  Rhinoceros,  and 
the  Lemming  (Myodes)  were  found  southern  animals,  such  as 
the  Hippopotamus,  several  kinds  of  Elephants  and  Rhinoceroses, 
Lions,  and  Hyaenas,  and  likewise  species  allied  to  those  still  liv- 
ing in  Europe,  such  as  the  huge  Cave  Bear,  the  gigantic  Irish  Deer 
(Megaeeros),  and  great  Oxen.  Northern  Africa  was  joined  to 
Europe  by  way  of  Malta  and  Sicily,  and  probably  at  Gibraltar 
also,  permitting  frequent  intermigrations.  The  junction  of  Ireland 
with  Great  Britain  and  of  both  with  the  continent  continued  until 
after  the  ice-sheets  had  disappeared,  so  that  these  islands,  and 
especially  Great  Britain,  were  stocked  by  the  continental  animals 
and  plants. 


MAN  539 

In  the  Pleistocene  of  India  are  found  many  animals  which  now 
live  only  in  Africa,  such  as  the  Baboon,  Spotted  Hyaena,  etc. 

Australia  had  a  Pleistocene  mammalian  fauna  composed,  with 
the  exception  of  the  Wild  Dog  (  Cam's  dingo),  of  Marsupials,  allied 
to  those  which  still  inhabit  that  region,  but  many  of  them  were  of 
vastly  greater  size  than  the  living  forms.^-*— -~" 

The  Pleistocene  mammals  are  rem^feable  for  the  great  size 
which  distinguishes  many  of  them,  and  it  is  just  these  which  have 
passed  away,  leaving  a  world  that  is  "  zoologically  impoverished," 
but  is  nevertheless  a  much  more  agreeable  place  of  residence  with- 
out them.  Further  we  note,  (i)  that  the  Pleistocene  mammals 
are  in  general  like  the  smaller  forms  which  have  succeeded  them 
in  the  same  regions,  but  (2)  that  in  Europe  and  North  America 
there  was  a  commingling  of  types  now  found  only  in  widely  sepa- 
rated regions. 

Man  first  appears  in  Europe  in  Glacial  times  ;  there  is  no  known 
reason  why  he  should  not  have  existed  in  North  America  at  the 
same  time,  but  as  yet  convincing  proof  of  his  presence  here  has 
not  been  obtained.  In  the  Recent  epoch  his  works  of  art  become 
numerous,  but  here  the  science  of  Anthropology  begins. 

We  have  now  taken  a  very  brief  and  hurried  survey  of  the 
earth's  history  from  its  beginning  as  a  nebula  to  the  condition  in 
which  we  find  it  to-day.  The  story  of  millions  of  years  has  been 
compressed  into  a  few  pages,  and  in  this  compression  it  is  impos- 
sible that  the  history  should  not  have  suffered  some  distortion. 
Events  widely  separated  in  time  and  space  are  brought  close  to- 
gether, just  as  two  stars  that  are  really  separated  by  well-nigh  infinite 
distances  may  seem  to  touch.  Yet  even  from  an  imperfect  out- 
line sketch  certain  great  truths  may  be  learned.  We  see  that 
the  whole  development  of  the  earth  has  been  under  the  domain 
of  law,  that  events  do  not  happen  capriciously  or  by  chance, 
but  in  an  orderly,  definite  way,  and  for  good  and  sufficient  reasons. 
True,  the  earth  is  but  a  very  small  member  of  the  Solar  System, 
and  the  latter  an  inconsiderable  member  of  the  Universe,  so 
that  we  might  be  tempted  to  think  of  the  earth  as  such  a  mere 


540  CONCLUSION 

speck  that  its  history  cannot  be  of  much  significance.  But  this 
would  be  to  confuse  greatness  with  mere  bigness,  to  exalt  Siberia 
above  Greece  in  the  development  of  mankind.  It  is  as  the  mani- 
festation and  theatre  of  intelligence,  of  conscious  life,  that  the 
earth  possesses  importance  and  real  significance,  however  tiny 
it  may  appear  when  compared  with  the  inconceivable  vastness  of 
the  Universe.  The  obvious  lesson  of  the  whole  history  is  that 
of  progress  and  development,  not  only  of  the  globe  itself,  but 
of  the  living  things  upon  it,  the  lower  giving  way  to  the  higher, 
the  simple  to  the  complex.  Last  of  all  appears  Man,  "  the  he'r 
of  all  the  ages,"  himself  the  crowning  work  of  progress,  who  alone 
of  living  beings  has  been  able  in  large  measure  to  emancipate 
himself  from  the  tyranny  of  natural  forces.  But  if  this  emancipa- 
tion is  to  justify  itself  and  prove  no  mere  mockery,  it  must  result 
not  simply  in  material  improvements,  but  in  advancement  and 
progress  along  all  lines  that  shall  lift  the  race  to  a  higher  plane 
and  make  it  worthy  of  its  opportunities  and  of  the  age-long  prepa- 
ration for  its  coming. 

"  The  solid  earth  whereon  we  tread 

"  In  tracts  of  fluent  heat  began 
And  grew  to  seeming-random  forms, 
The  seeming  prey  of  cyclic  storms, 
Till  at  the  last  arose  the  man; 

"  Who  throve  and  branch'd  from  clime  to  clime, 
The  herald  of  a  higher  race, 
And  of  himself  in  higher  place, 
If  so  he  type  this  work  of  time. 
****** 

" Arise  and  fly 

The  reeling  Faun,  the  sensual  feast; 
Move  upward,  working  out  the  beast, 
And  let  the  ape  and  tiger  die." 


APPENDIX    I 


In  order  to  avoid  the  necessity  of  correlating  the  American  and 
foreign  geological  subdivisions,  comparatively  few  names  of  the 
latter  have  been  mentioned  in  the  text,  because  such  correlations, 
in  the  present  state  of  knowledge,  can  seldom  be  made  with  con- 
fidence. Subjoined  are  tables  of  the  more  important  European 
formations. 

CAMBRIAN   SYSTEM 

SCANDINAVIA 


ENGLAND 

Upper  Cambrian, 
Olenns  or  Dicellocephalus 

Beds. 
Middle  Cambrian 

or 

Paradoxides  Beds. 
Lower  Cambrian 

or 
Olenelhts  Beds. 


Lingula  Flags. 


Menevian  and 
Solva. 


Caerfai. 


ORDOVICIAN  SYSTEM 


ENGLAND 

Bala. 

Llandeilo. 

Llanvirn. 

Arenig. 

Tremadoc. 


ENGLAND 

Ludlow. 

Wenlock. 

Llandovery. 


SILURIAN  SYSTEM 


541 


Olenus  Shales. 

Paradoxides  Beds. 

Fucoid  and 
Eophyton  Sandstone. 

SCANDINAVIA 
Brachiopod  Shales. 
Trinucleus  Shales. 

Chasmops  and 
Cystidean  Limestones. 

Orthoceras  Limestone. 
Ceratopyge  Limestone. 

SCANDINAVIA 

Gotland  Limestone. 


542 


APPENDIX 


DEVONIAN   SYSTEM 


Upper 
Devonian 


Middle 
Devonian 


Devonian 


ENGLAND 
Cypridina  Slates. 
Gonialite  Limestones 

and  Shales. 
I  Massive  Limestones, 
f  Middle  Devonian 

Limestones. 
Ashpringian  Vol- 

canics. 

Eifelian    Slates    and 
Limestones. 

•{  Grits  and  Sandstones. 


SCOTLAND 


GERMANY 


Clymenia  Limestones  and 

Cypridina  Slates. 
Goniatite  Limestones. 


f  Stringocephalus  Beds. 
L  Calceola  Beds. 


Coblenz  Beds. 
Hunsriick  Slates. 
Sericitic  Phyllite. 


CARBONIFEROUS   SYSTEM 


ENGLAND 

f  Upper  (Ardwick)  Series. 
Coal       I  Middle  Series. 
Measures  |  Lower  (Canister)  Series. 

I  Millstone  Grit. 
Lower      f  Yoredale  Series. 
Carbonif-  \  Scar  or  Mountain  Limestone, 
erous      [  Lower  Limestone  Shales. 


GERMANY 


[Ottweiler  Beds. 
ISaarbriick  Beds. 


Culm. 


GERMANY 


Rhsetic. 


Keuper. 


Muschelkalk. 


Bunter  Sandstone. 


PERMIAN  SYSTEM 

GERMANY 
Zechstein. 
Rothliegendes. 

TRIASSIC   SYSTEM 

MEDITERRANEAN   REGION 
Bajuvaric  J  Rhaetic  Stage. 

Series     [  Juvavian  Stage. 
Tirolic 

Series 
Dinaric 

Series 
Scythic 

Series 


f  Carinthian  Stage. 
[  Norian  Stage. 
J  Upper  Muschelkalk. 
(  Lower  Muschelkalk. 

Werfen  Beds. 


APPENDIX 


543 


Oolite 


JURASSIC  SYSTEM 

ENGLAND 

J  Portland. 
Uppet    [  Kimmeridge  Clay. 

f  Coral  Rag. 
Middle  I  Oxford  Clay. 

I  Kellaways  Rock. 

J  Great  Oolite. 
L°Wer    I  Inferior  Oolite. 


GERMANY 


Malm. 


I  Dogger. 
Lias. 


CRETACEOUS  SYSTEM 


ENGLAND 


f  Upper. 


Chalk 


Upper  |  Middle. 

Cretaceous  1  I  Lower.        1 

[  Upper  Greensand.     J 

I  Gault. 
Lower  Cretaceous     -I  Lower  Greensand. 

I  Wealden  and  Purbeck. 


FRANCE 

Danian. 

Senonian. 
Turonian. 

Cenomanian. 

Albian. 
Aptian. 
Neocomian. 


ENGLAND 

Chillesford  Beds. 
Norwich  Crag. 
Red  Crag. 
Coralline  Crag. 


TERTIARY   SYSTEM 
Pliocene 

ITALY 

Arno  Stage. 
Piacentine  Stage. 
Zancleano  Stage. 


FRANCE 

Marine  Fresh   Water 

Pontian.  Sansan. 

Sarmatian. 
Tortonian. 
Langhian 


Miocene 


FRESH   WATER 


St.  Gerand  le  Puy. 


Pikermi,  Attica. 
Mt.  Leberon,  France. 

VIENNA  BASIN 

Pontian  Stage. 

Sarmatian  Stage. 

Second  Mediterranean  Stage. 

First  Mediterranean  Stage. 


544 


APPENDIX 


Oligocene 

PARIS  BASIN 

f  Fresh-water  Limestone  of  Beauce  and 
Tongrian  -I       Upper  part  of  Fontainebleau  Sandstone. 
Fontainebleau  Sandstone. 

Ludian.       Gypsum  of  Montmarte. 


NORTH   GERMANY 
Sternberg  Rock. 
Sands  of  Cassel. 
Septaria  Clay  and 
Stettin  Sand. 
Egeln-Latdorf  Clays. 


Eocene 


PARIS   BASIN 

Sand  of  Beauchamp  and 

Limestone  of  St.  Ouen. 

Calcaire  grossier. 

Sand  of  Cuise. 

Clay  and  Lignite  of  Soissons. 

Sand  of  Bracheux. 
Marl  of  Meudon. 


LONDON   BASIN 

Lower  Headon  Beds. 
Barton  Clay. 
Bagshot  and 
Bracklesham  Beds. 
Woolwich  and  Read- 
ing Beds. 
Thanet  Sand. 


APPENDIX    II 

For  convenience  of  reference,  the  system  of  classification  of  the 
animals  and  plants  which  has  been  used  in  the  book  is  here  given 
in  tabular  form,  omitting  those  groups  which  possess  no  importance 
as  fossils.  Groups  marked  with  an  asterisk  (*)  are  extinct. 

ANIMALS 

A.   PROTOZOA 
RHIZOPODA 

Foraminifera 
Radiolaria 

B.   METAZOA 

I.   SPONGIDA,  Sponges 
II.   CCELENTERATA 
Class    i.  Hydrozoa 

a.  Hydroidea,  b.  Siphonophora,  c.  Discophora^  Jelly-fishes 
Class    2.  Anthozoa 

a.  Alcyonaria,  b.  Zoantharia,  Corals 
a.  Tetracoralla 
/3.  Hexacoralla 
Class    3.  Ctenophora 

III.  VERMES,  Worms 

IV.  ECHINODERMATA 
Class  *i.  Cystidea 
Class  *2.  Blastoidea 

Class    3.  Crinoidea,  Crinoids 

*a.  Palceocrinoidea  (Tessellata),  b.  Neocrinoidea  (Articulata) 
Class    4.  Asteroidea 

a.  Ophiurida,  Brittle-stars,  b.  Stellerida,  Star-fishes 
Class    5.  Echinoidea,  Sea-urchins 

Subclass  A.  *PAL^EOECHINOIDEA 
Subclass  B.     EUECHINOIDEA 

Order  a.   Regulares,  Regular  Sea-urchins 
Order  b.   Irregulares,  Spatangoids,  Sand-dollars 
Class    6.  Holothuroidea,  Sea-cucumbers. 
2N  545 


546  APPENDIX  II 

V.  ARTHROPODA 
Class  i.  Crustacea 

Subclass^.  *TRILOBITA 
Subclass  B.    GIGANTOSTRACA 

a.  *  Eurypterida  (Merostomata) 

b.  Xiphosura  (Limuloidea),  Horseshoe-crabs 
Subclass  C.     ENTOMOSTRACA 

a.  Ostracoda,  b.  Phyllopoda,  c.  Copepoda, 

d.  Cirripedia,  Barnacles 
Subclass  D.    MALACOSTRACA 
Order  a.  Euphausiacea 
Order  b.  Mysidacea 
Order  c.  Decapoda 

Suborder  a.  Macrura,  Lobsters,  etc. 
Suborder  /3.  Anomura,  Hermit-crabs,  etc. 
Suborder  7.  Brachyura,  Crabs 
Order  d.  Cumacea 
Order  e.  hopoda 
Order  f.  Amphipoda 
Order  g.  Stomatopoda 

Class  2.  Arachnoidea,  Spiders  and  Scorpions 
Class  3.  Myriapoda,  Centipedes 
Class  4.  Insecta,  Insects 

Order  a.  Orthoptera,  Cockroaches,  Grasshoppers,  etc. 

Order  b.  Neuroptera,  Caddis-flies,  Ant-lions,  etc. 

Order  c.  Hemiptera,  Cicadas,  etc. 

Order  d.  Diptera,  Flies 

Order  e.  Lepidoptera,  Butterflies  and  Moths 

Order/  Coleoptera,  Beetles 

Order  g.  Hymenoptera,  Bees,  Wasps,  Ants,  etc. 

VI.   BRYOZOA,  Sea-mosses 

VII.   BRACHIOPODA,  Lamp-shells 
Order  «.  Articulata 
Order  b.  Inarticulata 

VIII.   MOLLUSCA 

Class  i.  Pelecypoda,  Bivalves 

Order  a.  Asiphonida,  Oysters,  Mussels,  etc. 
Order  b.  Siphonida^  Clams,  etc. 
Class  2.  Glossophora 

Subclass  A.  SCAPHOPODA,  Tusk-shells 
Subclass  B.  PLACOPHORA,  Chitons 


APPENDIX  II  547 

Subclass  C.  GASTROPODA,  Univalves 

Order  a.  Prosobranchia,  Conches,  Whelks,  Cowries,  etc. 
Order  b.  Heteropoda 

Order  c.  Opisthobranchia,  Sea-slugs,  etc. 
Order  d.  Pulmonala,  Snails,  etc. 
Subclass  D.  PTEROPODA,  Pteropods 
Class  3.  Cephalopoda 
Order  a.  Tetrabranchiata 

Suborder  a.    Nautiloidea,  Nautilus,  etc. 
Suborder  ft.  *Ammonoidea,  Ammonites 
Order  b.  Dibranchiata 

Suborder  a.  Decapoda,  Cuttle-fishes,  etc. 
Suborder  /3.  Octopoda,  Octopus,  etc. 

IX.   VERTEBRATA 

Class  i.  Cyclostomata,  Lampreys 

?  *  Ostracodermata 
Class  2.  Pisces,  Fishes 

Subclass  A.  SELACHII,  Sharks  and  Rays 

Subclass  B.  HOLOCEPHALI,  Chimaeroids  or  Spook-fishes 

Subclass  C.  DIPNOI,  Lung-fishes 

Order  a.  Sirenoidei,  Existing  Lung-fishes 
Order  b.  ?  *  Arthrodira 
Subclass  D.  TELEOSTOMI 

Order  a.  Crossopterygii 
Order  b.  Actinopterygii 

Suborder  a.  Chondrostei  or  Ganoids,  Sturgeon, 

Gar-pike,  etc. 
Suborder  0.  Teleocephali  or   Teleosts,  Herring, 

Salmon,  etc. 
Class  3.  Amphibia 
Order  a.  * Stegocephala 

Order  b.  Gymnophiona,  Snake-like  Amphibians 
Order  c.  Urodela,  Mud-puppies,  Salamanders 
Order  d.  Anura,  Frogs  and  Toads 
Class  4.  Reptilia,  Reptiles 
Order  a.  *Proganosauria 
Order  b.     Rhynchocephalia 
Order  c.  * Ichthyosauria 
Order  d.  * Plesiosauria 

Order  e.     Testudinata,  Turtles  and  Tortoises 
Order  f.  *  Theromorpha 
Order  g.    Lepidosauria 


548  APPENDIX   II 

Suborder  a.    Lacertilia,  Lizards 
Suborder  j3.  * Pythonomorpha 
Suborder  7.     Ophidia,  Snakes 
Order  h.     Crocodilia,  Crocodiles 
Order  i.  *Dinosauria 
Order  k.  *Pterosauria 
Class  5.  Aves,  Birds 

Order  a,  *Saurura  (Archseopteryx) 
Order  b.    Ratitce,  Wingless  Birds,  Ostriches,  etc. 
Order  c.     Carinata,  Flying  Birds,  Eagles,  Sparrows,  Doves,  etc. 
Class  6.  Mammalia,  Mammals 

Subclass  A.  PROTOTHERIA 

Order  a.  Monotremata,    Spiny   Ant-eater,    Duck-billed 

Mole 

Order  b.  ?  *Multituberculata 
Subclass  B.  METATHERIA 

Order  a.  Marsupialia,  Opossums,  Kangaroos,  etc. 
Subclass  C.  EUTHERIA,  Placentals 

Order   a.    Edentata,  Sloths,  Armadillos,  Ant-eaters 

Order    b.     Cetacea,  Whales,  Dolphins 

Order    c.    Sirenia,  Sea-cows,  etc. 

Order   d.    Insectivora,  Moles,  Shrews,  Hedgehogs,  etc. 

Order    e.     Cheiroptera,  Bats 

Order  f.  *Creodonta 

Order  g.     Carnivora,  Dogs,  Cats,  Weasels,  Bears, 

Seals,  etc. 

Order   h.  *  l^illodonta 

Order    z.    Rodentia,    Squirrels,   Beavers,   Mice,   Porcu- 
pines, Rabbits,  etc. 
Order  /.  *  Condylarthra 
Order    k.  *Amblypoda 
Order    /.  *  Ty pother ia 
Order  m.  *Litopterna 
Order  n.  *  Toxodontia 
Order    o.    Proboscidea,  Elephants 

Order  /.    Artiodactyla,  Swine,  Camels,  Deer,  Oxen,  etc. 
Order   q.    Perissodactyla,  Horses,  Tapirs,  Rhinoceroses 
Order   r.    Lemur oidea,  Lemurs 
Order    s.    Primates,  Monkeys,  Apes,  Man 


APPENDIX  II  549 


PLANTS 

I.  THALLOPHYTA 

Class  i.  Algae,  Seaweeds,  etc. 
Class  2.  Fungi,  Mushrooms,  etc. 

II.   BRYOPHYTA 

Class  i.  Anthoceroteae 

Class  2.  Hepaticae,  Liverworts 

Class  3.  Musci,  Mosses 

III.   PTERIDOPHYTA 
Class  i.  Filices,  Ferns 
Class  2.  Rhizocarpeae,  Pepperworts 
Class  3.  Equisetaceae  (Calamaria)  ,  Horsetails 
Class  4.  Lycopodiaceae,  Club-mosses 

PHANEROGAMS,  Flowering  Plants 

IV.  GYMNOSPERM^: 

Order  a.  Cycadacea,  Cycads,  Sago-palms 


Order  b.  Conifera,  Pines,  Spruces,  Cypresses,  etc. 

V.  ANGIOSPERM^ 

Class  i.  Monocotyledones,  Grasses,  Lilies,  Palms,  etc. 
Class  2.  Dicotyledones,  Oaks,  Elms,  etc. 


INDEX 


An  asterisk  (*)  after  a  page  number  indicates  that  a  figure  of  the  object  named  will  be  found 
on  that  page,  while  a  dagger  (f)  implies  that  a  definition  or  explanation  is  contained  in  the 
page  designated,  though  this  symbol  is  not  employed  unless  there  is  more  than  one  reference 
under  a  given  head.  Family  and  generic  names  of  animals  and  plants  are  printed  in  Italics. 


ACADIAN  COAL  FIELD,  414. 

Acadian  epoch,  355,  368. 

Acadian  province,  395f,  428. 

Acadian  range,  333. 

Acanthodes,  426. 

Accessory  minerals,  193. 

Accidents  to  rivers,  330. 

Acervularia,  400,  401*. 

Acid  lavas,  42;  rocks,  191,  195,  196. 

Acidaspis,  400. 

Acquia  Creek  stage,  475. 

Actinocrinus,  400,  424. 

Actinolite,  20. 

Actinopterygii,  Carboniferous,  406 ;  De- 
vonian, 426. 

Adjustment  of  rivers,  321,  326f. 

Adjutants,  fossil,  516. 

Adriatic  Sea,  514. 

^Egean  Sea,  514,  515. 

sfcgoceras,  467. 

^Eolian  rock,  I27f,  2O5f,  217. 

sEsiocrhtus,  423*,  424. 

.'Etna,  98  ;  dikes  in,  50 ;  origin  of,  520. 

Aetosaurus,  454. 

Africa,  Archaean  of,  360;  Carboniferous 
of,  418  ;  Cretaceous  of,  484 ;  Devonian 
of,  399 ;  Eocene  of,  500 ;  Jurassic  of, 
461 ;  Miocene  of,  514 :  Ordovician  of, 
377  ;  Permian  of,  437  ;  Silurian  of,  388  ; 
Triassic  of,  448. 

Aftonian  stage,  529,  53of. 

Agassiz,  A.,  173. 

Agate,  16. 

Agathaumas ',  492*. 


Age,  geological,  354. 

Agelacrinus,  383*. 

Agglomerate,  volcanic,  5 if,  2O3f,  277. 

Agnostus,  372,  373*. 

Alabaster,  23. 

Alaria,  466. 

Alaska,  Eocene  flora  of,  502 ;  glaciation 
°f,  527 ;  glaciers  of,  108 ;  Jurassic  of, 
461 ;  Miocene  of,  512 ;  Pliocene  of,  518. 

Albertan  drift  sheet,  529. 

Albirupean  stage,  475. 

Albite,  i6f,  17. 

Aleutian  Islands,  work  of  frost  in,  83. 

Algae,  calcareous,  450 ;  deposits  by,  130. 

Algonkian  period,  355,  36if. 

Alkali  streams,  102. 

Alkaline  carbonates,  128,  130,  267;  sul- 
phides, 130,  267. 

Allodon,  493. 

Allorisma,  423*,  425. 

Allotriomorphic  crystals,  190. 

Alluvial  cone,  i37t,  138*,  231 ;  fan,  137. 

Aloes,  fossil,  502. 

Alpine  glaciers,  no. 

Alps,  325,  333,  334,  496 ;  glaciers  of,  107 ; 
Pleistocene  glaciers  0^536;  upheaval 
of,  514,  515. 

Altai  Mountains,  399. 

Alteration  of  rocks,  287. 

Amazon  River,  102. 

Amblypoda,  505. 

Ambonychia,  383*. 

Amethyst,  15. 

Ammonites,  Eocene,  503. 


551 


552 


INDEX 


Ammonoidea,  4O2f,  Carboniferous,  426; 
Cretaceous,  486;  Devonian,  402;  Ju- 
rassic, 466;  Mesozoic,  442;  Permian, 
434 ;  Triassic,  452. 

Amorphous  minerals,  n. 

Amphibia,  Carboniferous,  427;  Cenozoic, 
495  ;  Devonian,  407  ;  Jurassic,  469  ; 
Mesozoic,  442;  Palaeozoic,  367;  Per- 
mian, 435 ;  Triassic,  453. 

Amphiboles,  igf,  194,  289. 

Amphitheatres,  316. 

Amp  hither  turn,  473. 

Amygdaloidal  texture,  190. 

Analcite,  i8f,  202. 

Anaptoinorphus,  505. 

Anchisaurus,  455. 

Anchura,  489*. 

Ancodus,  509. 

Ancyloceras,  488. 

Andes,  Pleistocene  glaciers  of,  536;  up- 
heaval of,  483. 

Andesine,  i6f,  17. 

Andesite,  2oof,  279,  297 ;  breccia,  203 ; 
obsidian,  200;  tuffs,  203. 

Angiospermae,  Mesozoic,  441. 

Anhydrite,  23. 

Annular ia,  421. 

Anomodontia,  455. 

Anoplotheres,  509. 

Anorthic  system  of  crystals,  10. 

Anorthite,  i6f,  201. 

Anorthosite,  202. 

Antarctic  continent,  Eocene,  501;  ice- 
sheet  of,  108. 

Ant-eaters,  fossil,  523,  538. 

Antecedent  rivers,  324. 

Antelopes,  fossil,  517,522. 

Anthozoan  corals,  463. 

Anthracite,  2i6f,  289,  295,  414. 

Anthracopalcemon ,  424. 

Anthracotherium,  509. 

Anticlinal  ridges,  318,  319 ;  valleys,  319. 

Anticline,  234*,'  235*. 

Anticlinorium,  236*. 

Anticosti  Island,  378. 

Ants,  fossil,  442,  465. 

Apatite,  24^  193,  198,  199. 

Apennines,  upheaval  of,  515. 

Aphelops,  517*. 

Apiocrinus,  464. 

Appalachian  coal  field,  414. 


Appalachian  land,  369,  375,  386,  387. 

Appalachian  Mountains,  237,  334 ;  cycles 
in,  341 ;  denudation  of,  103 ;  thrust 
faults  of,  254 ;  upheaval  of,  439. 

Appalachian  range,  332. 

Appalachian  revolution,  439. 

Aqueous  rocks,  205. 

Aragonite,  23f,  171,  214,  290. 

Aralia,  515. 

Araucanian  formation,  series,  or  stage, 

523-  524. 

Araucarians,  Jurassic,  462 ;  Triassic,  449. 

Araucarites,  449. 

Arbor  Vitae,  502. 

Area,  434. 

Arcestes,  451*,  452. 

Archaean  period,  355,  358^ 

Archaean  rocks,  theories  as  to,  360. 

Archceopteryx,  473*,  492. 

Archcgosaurus,  427. 

Archimedes,  419*,  425. 

Arctic,  flora  and  fauna,  537;  shells  in 
England,  522. 

Arenaceous  shale,  208. 

Argillaceous  rocks,  207 ;  sandstone,  206. 

Arietites,  467. 

Arizona,  earthquake  of  1887,  63. 

Arkose,  2o6f,  293. 

Armadillos,  fossil,  522,  523,  538. 

Artesian  wells,  94. 

Arthrodira,  Carboniferous,  426;  Devo- 
nian, 405f. 

Arthropoda,  Cambrian,  372;  Carbonif- 
erous, 424;  Cretaceous,  488;  Devo- 
nian, 400;  Jurassic,  464;  Mesozoic, 
442;  Ordovician,  381;  Permian,  432; 
Silurian,  391 ;  Triassic,  450. 

Articulata,  374. 

Artiodactyla,  505,  506,  509. 

Arto carpus,  515. 

Asaphus,  381,  383. 

Asar,  157,  3i2f. 

Asbestus,  20. 

Ashes,  volcanic,  5if,  203. 

Asia,  Archaean  of,  360;  Cambrian  of, 
370 ;  Carboniferous  of,  417 ;  Creta- 
ceous, 484;  Devonian  of,  398,  399, 
Eocene,  500;  Jurassic,  461 ;  Miocene, 
514;  Ordovician,  377;  Permian,  431 ; 
Pleistocene,  536 ;  Silurian,  388  ;  Trias- 
sic, 448. 


INDEX 


553 


Aspidorhynchus,  468*. 

Asteroidea,  381. 

Astral  period,  357. 

Astrophyllites,  421. 

Astylospongia,  389,  392*. 

Asymmetric  system  of  crystals,  10. 

Asymmetrical  folds,  237*,  239. 

Athyris,  401*,  402,  451. 

Atlantosaurus  beds,  477. 

Atlas  Mountains,  484,  496. 

Atmosphere,  destructive  work  of,  72. 

Atolls,  170. 

Atrypa,  393,  425. 

Aturia,  504*. 

Aucella,  488,  489*. 

Augite,  2of,  198,  202,  289,  297 ;  crystals 
from  Stromboli,  47. 

Augite  andesite,  200 ;  granite,  198 ;  sye- 
nite, 199. 

Augitite,  202. 

Auriferous  gravels  of  California, 496,512!. 

Au  Sable  Chasm,  99*,  100,308. 

Austin  Limestone,  475. 

Australia,  Archaean  of,  360;  Cambrian, 
370;  Carboniferous,  418 ;  Cretaceous, 
484;  Eocene,  501 ;  Miocene,  515;  Or- 
dovician,  377;  Permian,  436;  Pleisto- 
cene mammals  of,  539 ;  Silurian  of,  389. 

Australian  Alps,  Pleistocene  glaciers  of, 
536. 

Australian  barrier  reef,  167*,  168*,  170. 

Avalanches,  105,  106. 

Aviculopecten ,  423*,  425,  433*,  434. 

Axes  of  crystals,  9. 

Azoic  era,  355. 

BABOON,  fossil,  539. 

Bad  rites,  403. 

Baculites,  489*,  490. 

Bad  lands,  78,  79*,  313,  317*,  505,  507. 

Baiera,  432,  449. 

Bajuvaric  series,  443,447. 

Bald  Mountain,  256. 

Banded  veins,  266. 

Banks,  limestone,  172. 

Baptanodon,  470. 

Barite,  266. 

Barnacles,  381. 

Barren   Measures,  Lower,  409;    Upper, 

428,  429. 
Barrier  reefs,  170. 


Barriers,  land,  353. 

Barus,  C.,  43. 

Basal  complex,  358. 

Basal  conglomerates,  271. 

Basalt,  2oif,  279, 284 ;  family,  200 ;  joints 
of,  262 ;  leucite,  201 ;  nepheline,  201 ; 
olivine,  201 ;  quartz,  201. 

Basaltic  breccia,  203  ;  tuffs,  203. 

Base-level,  73,  98f,  302;  local,  309;  tem- 
porary, 309. 

Basement  complex,  358. 

Basin,  235. 

Basin  Ranges,  337*;  upheavals  of,  519, 534. 

Bats,  fossil,  506. 

Beaches,  119,  306;  raised,  67,  534. 

Bears,  fossil,  517,  522,  538. 

Beaver,  fossil,  509 ;  giant  fossil,  538. 

Beaverdam  Creek,  328. 

Bedding,  cross,  223*,  224 ;  current,  224 ; 
false,  224 ;  horizontal  and  oblique,  273. 

Bedding  planes,  146. 

Beeches,  fossil,  486,  502,  515,  516. 

Beechy,  Capt.,  83. 

Bees,  fossil,  442,  465. 

Beetles,  fossil,  442,  465. 

Beheading  of  streams,  328,  329*. 

Belemnitella,  489*,  490. 

Belemnites,  442,  452,  467,  490,  495,  503. 

Belemnites,  463*.  467!. 

Belleiophon,  419*.  425.  434. 

Belly  River  stage,  475,  48if. 

Belodon,  454*. 

Benton  substage,  475,  48lf. 

Bermuda,  shell  sands  of,  127. 

Betulites,  485*. 

Binary  granite,  198. 

Biotite,  I9f,  193,  198,  289. 

Biotite  andesite,  200,  gneiss,  296. 

Birds,  Cenozoic,  495 ;  Cretaceous,  492 ; 
Eocene,  503 ;  Jurassic,  472 ;  Miocene, 
Si6. 

Bisons,  fossil,  538. 

Bituminous  coal,  216,  shale,  208. 

Bivalves,  Cambrian,  374 ;  Carboniferous, 
425  ;  Cenozoic,  495  ;  Cretaceous,  488  ; 
Devonian,  402;  Eocene,  502;  Jurassic, 
465  ;  Mesozoic,  442  ;  Ordovician,  382 ; 
Permian,  434;  Triassic,  451. 

Black  Hills,  199 ;  Algonkian  of,  363  ;  Cre- 
taceous of,  477 ;  Devonian  of,  397 ; 
Jurassic  of,  461 ;  Silurian  of,  387. 


554 


INDEX 


Black  Jura,  see  Lias. 

Blanco  stage,  496,  5i8f. 

Blastoidea,  422!;  Carboniferous,  422; 
Devonian,  400 ;  Palaeozoic,  367 ;  Silu- 
rian, 390. 

Blastomeryx,  516. 

Block  Island  clays,  164. 

Blocks,  erratic  or  perched,  154*,  155*; 
volcanic,  51. 

Blown  sand,  125,  217. 

Blue  mud,  175. 

Blue  Ridge,  328,  333,  369. 

Bog  iron-ore,  I35t,  2i5f. 

Bombs,  volcanic,  51. 

Bony  Fishes,  490. 

Borneo,  Pliocene  of,  520. 

Bosses,  284. 

Boulder  beds,  Permian,  436. 

Brachiopoda,  Cambrian,  373;  Carbonif- 
erous, 425;  Cretaceous,  488;  Devo- 
nian, 402;  Jurassic,  465;  Mesozoic, 
442;  Ordovician,382;  Palaeozoic,  367  ; 
Permian,  434 ;  Silurian,  391 ;  Triassic, 
450. 

Brachiospongia,  383*. 

Brachyura,  Cretaceous,  488 ;  Eocene, 
503 ;  Jurassic,  464. 

Brahmapootra,  delta  of,  141,  142. 

Brain-casts,  346. 

Rranchiosaurus,  427. 

Breadfruit,  fossil,  515. 

Break  thrust,  253*. 

Breccia,  125,  2o6f,  217;  coral,  i68f;  vol- 
canic, 5  if,  2O3f. 

Brick  clay,  207. 

Bridger  stage,  496,  499,  506;  substage, 
496. 

Bridges,  land,  353. 

Brittle  Stars,  Jurassic,  464 ;  Ordovician, 
381. 

Brogniart,  457,  501. 

Bronzite,  20. 

Brown  coal,  2i5f ;  Oligocene,  507. 

Brown  Jura,  457. 

Bryozoa,  Carboniferous,  425 ;  Ordovi- 
cian, 382;  Permian,  432;  Silurian, 
391 ;  Triassic,  450. 

Buchanan  stage,  529. 

Bunter  Sandstone,  447. 

Buried  forests,  67. 

Burlington  substage,  409. 


Butte,  80,  313. 
Butterflies,  fossil,  442,  465. 
Buzzards,  fossil,  503. 

C^NOPUS,  509. 

Cairngorm,  15. 

Calamites,  420,  423. 

Calcareous  Algae,  Triassic,  450. 

Calcareous    minerals,    22;    shale,   208; 
sinter,  209 ;  tufa,  150*.  2O9f. 

Calciferous  stage,  375,  3761% 

Calcite,  22f,  266,  289,  290,  295. 

Callipteris,  432,  433*. 

Calymene,  381,  383*,  391. 

Cambrian  period,  355,  368. 

Camels,  fossil,  506,  509,  517,  522. 

Camptosaurus,  491. 

Canadian  epoch,  355  ;  series,  375. 

Cants  dingo,  539. 

Cannel  coal,  216. 

Canons,  308. 

Caprotina,  488,  489*. 

Capture  of  streams,  326,  329*. 

Capulus,  392*,  393. 
Carboniferous  period,  355,  4o8f. 

Cardita,  451,  504*. 
Carnivora,  506,  508,  516,  517. 
Carolina  ridge,  511. 

Cassidulus,  487. 

Castoroides,  538. 

Casts,  fossil,  346. 

Catastrophism,  doctrine  of,  351. 

Catfishes,  fossil,  490. 

Cats,  fossil,  522,  538. 

Catskill  series,  397,  399. 

Caturus,  468. 

Caucasus,  496,  515. 

Caulopteris,  449. 

Cave  Bear,  538. 

Cave  deposits,  131;  earth,  132. 

Caverns,  86,  89f. 

Cavies,  fossil,  538. 

Cementation,  289. 

Cementing     material     of     sediments, 

183. 

Cenozoic  era,  355,  494f. 
Centipedes,  Ordovician,  382;  Palaeozoic, 

367. 
Central  America,  Miocene,  511 ;  Triassic, 

444,  447 ;  upheaval  of,  514.  g£ 
Cephalaspis,  403. 


INDEX 


555 


Cephalopoda,  Cambrian,  374;  Carbonif- 
erous, 425  ;  Cretaceous,  488 ;  Devo- 
nian, 402;  Jurassic,  466;  Mesozoic, 
442;  Ordovician,  382;  Permian,  434; 
Silurian,  393;  Triassic,  452. 

Ceratites,  452. 

Ceratodus,  435,  453,  467. 

Ccrlthium,  452. 

Cetiosaurus,  471,  491. 

Chtztetes,  425. 

Chain,  mountain,  333. 

Chaix  Hills,  533. 

Chalcedony,  15,  deposition  of,  130. 

Chalk,  2i2f,  213*. 

Chalybeate  springs,  130. 

Chamberlin,  T.  C.,  526,  529. 

Champlain  stage,  533. 

Changes,  climatic,  geographical,  how 
proven,  352,  353  ;  of  level,  64 ;  of  tem- 
perature, effect  upon  rocks,  85. 

Charleston  Mountains,  337*. 

Chattahoochee  stage,  496,  5iif. 

Chazy  stage,  375,  376^ 

Cheirodus,  426. 

C  heir  other  him,  453. 

Chemical  deposits,  127 ;  lacustrine,  I48f, 
I49f;  marine,  174^ 

Chemical  precipitates,  208. 

Chemung  epoch,  355 ;  series,  394,  396. 

Chert,  i6f,  210,  214. 

Chesapeake  stage,  496,  5iif. 

Chester  stage,  409. 

Chestnuts,  fossil,  486. 

Chico  series,  475,  483^ 

Chimaeroids,  467. 

China  clay,  207. 

China,  Liassic  coal  of,  460;  loess  of,  125. 

Chipola  stage,  496,  51  if. 

Chlorite,  2if,  90. 

Chlorite  schist,  298. 

Chonetes,  402,  419*,  425. 

Choristoceras,  452. 

Chronology,  geological,  221,  347,  349. 

Cidaris,  424,  487. 

Cincinnati  anticline,  378,  410 ;  stage,  375. 

Cinnabar,  130. 

Cinnamomum,  487*. 

Cirripedia,  381. 

Civet  cats,  508,  522. 

Cladoselache,  404*. 

Claiborne  stage,  496. 


Clark,  W.  B.,  532. 

Clathropteris,  449,  451*. 

Clay,  204t,  2O7f ;  red,  180. 

Clear  Fork  beds,  428,  43of. 

Cleavage,  of  minerals,  13  ;  of  rocks,  260, 
261*.  334 ;  cause  of,  261. 

Clidastes,  490*. 

Climate,  Carboniferous,  421 ;  Cenozoic, 
494;  Eocene,  506;  Jurassic,  461 ;  Meso- 
zoic, 443;  Miocene,  517;  Oligocene, 
509  ;  Palaeozoic,  368  ;  Pleistocene,  525  ; 
Pliocene,  522  ;  Triassic,  447. 

Climatic  changes,  evidences  of,  352; 
zones,  Jurassic,  461. 

Clinometer,  232*. 

Clinton  stage,  385,  386f. 

Closed  folds,  237*,  239*. 

Club  mosses,  Palaeozoic,  367. 

Clymenia,  403. 

Coal,  215  ;  Cretaceous,  477, 479,  481-485 ; 
Liassic,  460 ;  origin  of,  413 ;  Triassic, 
444,  447,  448. 

Coal  fields  of  North  America,  414. 

Coal  Measures,  409,  412;  False,  411. 

Coast  ice,  115  ;  deposits  by,  159. 

Coast  line,  changes  of,  65f,  305. 

Coast  Range,  332;  origin  of,  459;  up- 
heaval of,  459. 

Coccosteus,  405,  406*. 

Cochloceras,  452. 

Cod,  fossil,  490. 

Ccelacanthus ,  426. 

Coelenterata,  Cambrian,  371 ;  Carbonif- 
erous, 422 ;  Cretaceous,  487 ;  Devo- 
nian, 400;  Jurassic,  463  ;  Ordovician, 
379;  Permian,  432;  Silurian,  389;  Tri- 
assic, 450. 

Coleoptera,  465. 

Colorado  River,  Grand  Canon  of,  100. 

Colorado  Island,  415,  430,  445,  458. 

Colorado  stage,  475,  48  if. 

Colouring  of  rocks,  25. 

Columbian  formation,  532. 

Columnaria,  380. 

Columnar  joints,  262. 

Comanche  series,  475,  476f. 

Com  at  u  la,  464. 

Como  stage,  475,  4771. 

Compact  texture,  190. 

Complex,  basal  or  basement,  358. 

Compound  faults,  247. 


556 


INDEX 


Compression  joints,  265. 

Compression,  lateral,  255f,  336;    origin 

of,  337- 

Compsognathus,  471. 

Concretions,  22yt,  229*,  230*. 

Condylarthra,  505. 

Cones,  alluvial,  137,  138*;  volcanic,  54. 

Coney  Island,  waste  of,  118. 

Conformity,  269  ;  deceptive,  270. 

Conglomerate,  207 ;  basal,  271 ;  coral,  168. 

Coniferae,  Carboniferous,  421 ;  Creta- 
ceous, 485;  Devonian,  400;  Jurassic, 
462;  Miocene,  515;  Oligocene,  508; 
Permian,  432;  Triassic,  449. 

Conocardium,  401*,  402. 

Conocoryphe,  373*. 

Consequent  drainage,  323 ;  streams,  321. 

Consolidation  of  sediment,  182. 

Contact  metamorphism,  288. 

Contemporaneous  erosion,  272*. 

Contemporaneous  sheet,  2771%  282. 

Continental,  glaciers,  no;  platform,  160, 
162*. 

Contorted  folds,  240. 

Conularia,  419*,  42^.-. 

Copper,  266,  364 ;  deposition  of,  130. 

Coquina  rock,  171*. 

Coral  limestone,  213 ;  reefs,  166. 

CoralHochama,  488. 

Corals,  i65t  ;  Cambrian,  372  ;  Carbonif- 
erous, 422;  Cretaceous,  487;  Devon- 
ian, 400 ;  Jurassic,  463  ;  Mesozoic,  441 ; 
Ordovician,  379;  Palaeozoic,  367  ;  Per- 
mian, 432;  Silurian,  389 ;  Triassic,  450. 

Cordaites,  421,  432. 

Cordillera,  333. 

Cordilleran  ice-sheet,  526,  527,  531. 

Cormorants,  fossil,  492. 

Corniferous  epoch,  355;  series,  394; 
stage,  394,  396^ 

Coryphodon,  505. 

Cosoryx,  516. 

Cotopaxi,  51. 

Country  rock,  265. 

Crabs,  464,  488,  503. 

Cranes,  fossil,  516. 

Crater  Lake,  39* ;  ring,  39. 

Creep  of  shales,  82*. 

Creodonta,  505,  506,  508. 

Cretaceous  period,  355,  474t. 

Crete,  changes  of  level  in,  66. 


Crevasse  in  glacier,  107*. 

Crinoidal  limestone,  213. 

Crinoidea,  Cambrian,  372;  Carbonifer- 
ous, 422  ;  Cretaceous,  487  ;  Devonian, 
400;  Jurassic,  464;  Mesozoic,  441; 
Ordovician,  380 ;  Palaeozoic,  367;  Per- 
mian, 432;  Silurian,  390;  Triassic,  450. 

Crioceras,  488. 

Crocodiles,  Cenozoic,  495 ;  Cretaceous, 
491 ;  Eocene,  503  ;  Jurassic,  470  ;  Oligo- 
cene, 508 ;  Triassic,  454. 

Cross-bedding,  223*,  224!-;  faults,  248. 

Cross  Timber  Sand,  Lower,  475. 

Crossopterygii,  405-7,  426,  453,  467. 

Crust  of  earth,  8  ;  formation  of,  357. 

Crustacea,  Cambrian,  372;  Cretaceous, 
488;  Devonian,  400;  Jurassic,  464; 
Mesozoic,  442 ;  Ordovician,  381 ;  Silu- 
rian, 391. 

Cryptogams,  Palaeozoic,  367. 

Crystal,  definition  of,  9. 

Crystalline  rocks,  188. 

Crystallites,  46. 

Crystallization,  12 ;  of  rock  magmas,  192 ; 
systems  of,  9. 

Crystals,  compound,  14;  cruciform,  14; 
geniculate,  14 ;  physical  properties  of, 
n  ;  secondary  forms  of,  13 ;  twinned,  14. 

Ctenacodon,  493. 

Cfenodus,  426. 

Cube,  9*. 

Cubical  system  of  crystals,  9. 

Culm,  4i6f,  417. 

Cupressites,  462. 

Cupressocrinus,  400. 

Current  bedding,  224. 

Cycads,40o;  Carboniferous,  421 ;  Creta- 
ceous, 485  ;  Jurassic,  462 ;  Mesozoic, 
441;  Permian,  432;  Triassic,  449. 

Cycles  of  denudation,  3031,  340,  341. 

Cycloceras,  426. 

Cyclonema,  392*,  393. 

Cyclotosaurus,  453. 

Cynodictis,  506. 

Cynoglossa,  432. 

Cyprcea,  488,  516. 

Cypresses,  fossil,  502. 

Cyrtina,  451. 

Cystidea,  Cambrian,  372;  Devonian, 
400 ;  Ordovician,  380 ;  Palaeozoic,  367 ; 
Silurian,  390.  ^) 


INDEX 


557 


DACITE,  200. 

Dakota  stage,  475,  479t. 

Dalmafiites,  381,  390*,  391. 

Dainmarites,  485*. 

Dana,  J.  D.,  375,  408. 

Dapedius,  468*. 

Darwin,  Charles,  123. 

Davis,  W.  M.,  324. 

Dawson,  G.  M.,  527. 

Dead  Sea,  309. 

Decapoda,  424,  464. 

Deccan,  lava  fields  of,  58. 

Deep  River  substage,  496,  5i3f. 

Deep-sea  deposits,  174. 

Deer,  fossil,  517,  522,  524,  538. 

Degradation,  301. 

Delaware  River,  324,  327. 

Delaware  Water  Gap,  81*,  83,  308,  314. 

Deltas,  I4of,  141,  142,  143. 

Dendrerpeton,  427. 

Dendrocrinus,  383*. 

Denudation,  7if,  301,  307,  310,  339; 
cycles  of,  340. 

Denver  stage,  475,  482^ 

Deposits,  chemical,  127,  147,  148,  174; 
deep-sea,  174;  estuarine,  181;  fluviatile, 
136;  glacial,  153,  311;  ice,  153,  159, 
527;  iceberg,  158;  lake,  143;  littoral, 
162;  marine,  160;  mechanical,  205; 
organic,  133,  148,  166;  pelagic,  176; 
shallow-water,  164 ;  swamp,  133 ;  ter- 
restrial, 124;  terrigenous,  174. 

Depression,  65  ;  causes  of,  68  ;  evidences 
of,  67. 

Deserts,  denudation  in,  84. 

Destruction  of  rock,  71. 

Devil's  Tower,  283,  285*. 

Devitrification,  i2f,  197. 

Devonian  period,  355,  394f. 

Diabase,  20 if,  279,  297,  445. 

Diallage,  20. 

Diastrophism,  301. 

Diatom  ooze,  179,  180*. 

Diatoms,  148,  182,  214. 

Dibranchiata,  374,  442,  452,  467. 

Diceras,  466. 

Diclonius,  492*. 

Dicotyledons,  441,  485. 

Dicranograptus,  383*. 

Dicrocynodon,  493. 


Dicynodon,  455. 
Didelphops,  493. 
Dikes,  50,  278*1,  279* ;  sandstone,  268*, 

Dimetric  system  of  crystals,  9. 

Dinaric  series,  443,  447. 

Dinichthys,  405. 

Dinosauria,  454t,  471,  491,  495. 

Dinot her  turn,  517,  522. 

Diorite,  2Oof,  284  ;  family,  200. 

Dioritic  gneiss,  296. 

Dip,  232;  of  fault,  243;  initial,  231,  256, 
257*. 

Dip  faults,  246,  250,  251*. 

Dip  joints,  264. 

Dip  slope,  316. 

Diplodocus,  491. 

Diplograptus,  383. 

Diplurus,  453. 

Dipnoi,  404f,  426,  435,  453,  467. 

Diptera,  465. 

Dipterus,  405*. 

Discina,  373. 

Displacements  of  coast-line,  65  ;  of 
strata,  230. 

Diversion  of  streams,  328,  329*. 

Divides,  326;  shifting  of,  327. 

Dodecahedron,  9*. 

Dogs,  fossil,  508,  522. 

Dogger,  457. 

Dolerite,  201. 

Dolomite  (mineral),  23;  (rock),  213; 
crystalline,  294. 

Dolomitization,  I7of,  214. 

Dolphins,  fossil,  517. 

Dome,  235. 

Double  Mountain  Beds,  428,  430. 

Downthrow  (of  faults),  243. 

Dragon-flies,  fossil,  465. 

Drainage,  consequent,  323;  superim- 
posed, 326;  transferred,  329. 

Drakenbergen,  437. 

Drift,  527;  englacial,  114;  stratified, 
528. 

Drift-sand  rock,  127. 

Driftwood  theory  of  coal,  413. 

Dynamic  agencies.  30;  metamorphism, 
291. 

EAGLE  FORD  SHALES,  475. 
Eagles,  fossil,  503,  516, 


558 


INDEX 


Earth's  interior,  hypotheses  concerning, 
33 1  physical  state  of,  32 ;  temperature 
of,  31. 

Earthquakes,  61 ;  causes  of,  64 ;  distri- 
bution of,  61 ;  effects  of,  63  ;  phenom- 
ena of,  62. 

Earthworms,  geological  work  of,  123. 

Echinodermata,  Cambrian,  372;  Car- 
boniferous, 422;  Cretaceous,  487;  De- 
vonian, 400;  Eocene,  502;  Jurassic, 
464 ;  Mesozoic,  441 ;  Ordovician,  380 ; 
Palaeozoic,  367,  381 ;  Permian,  432 ; 
Silurian,  390;  Triassic,  450. 

Echinoderms,  modern  deposits  of, 
171. 

Echinoidea,  Carboniferous,  424 ;  Creta- 
ceous, 487;  Devonian,  400;  Eocene, 
502 ;  Jurassic,  464 ;  Mesozoic,  441 ; 
Ordovician,  380 ;  Palaeozoic,  367,  390 ; 
Silurian,  390;  Triassic,  450. 

Edentata,  523. 

Elephants,  fossil,  522,  538 ;  frozen  car- 
cases of,  345. 

Elements  of  earth's  crust,  8. 

Elevation  of  land,  65 ;  causes  of,  68 ; 
evidences  of,  66. 

Elk  Mountains,  284,  338. 

Elliptocephalus,  373*. 

Elm,  landslip  of,  92. 

Elms,  fossil,  486,  502,  515,  516. 

Elotherium,  509. 

Emarginula,  452. 

Embedding  of  fossils,  343. 

Encrinurus,  391. 

Encrinus,  450. 

Endoceras,  384,  393. 

Englacial  drift,  114. 

Eocene  epoch,  355,  497t;  series, 
496. 

Eohyus,  505. 

Eozoic  era,  355. 

Epeirogenic  diastrophism,  301. 

Epigenetic  drainage,  326. 

Epoch,  geological,  354. 

Equisetaceae,  Carboniferous,  420 ;  Creta- 
ceous, 485;  Devonian,  400;  Jurassic, 
462 ;  Mesozoic,  441 ;  Palaeozoic,  367 ; 
Triassic,  448,  449. 

Equisetum,  449. 

Equus  Beds,  532. 

Era,  geological,  354. 


Erosion,  71 ;  atmospheric,  72 ;  contem- 
poraneou^,  272*,  glacial,  no;  lake, 
120;  marine,  116;  river,  97 ;  tidal,  119. 

Erosion  thrust,  253.* 

Eruptive  rocks,  189,  274-^. 

Eryops,  435*. 

Escarpment,  314. 

Eskers,  312,  528. 

Essential  minerals,  193. 

Estuaries,  i8if,  306. 

Estuarine  deposits,  181. 

Eucalyptocrinus,  390. 

Euechinoidea,  424,  441,  450. 

Eugeniacrinus,  464. 

Euomphalus,  401*,  402,  423*,  425. 

Europe,  Algonkian  of,  363  ;  Archaean  of, 
360;  Cambrian  of,  370;  Carboniferous 
of,  416  ;  Cretaceous  of,  483  ;  Devonian 
of,  398  ;  Eocene  of,  500 ;  J  urassic  of, 
460;  Miocene  of,  514;  Oligocene  of, 
507  ;  Ordovician  of,  377,  378  ;  Permian 
of,  430 ;  Pleistocene  of,  535 ;  Pleisto- 
cene mammals  of,  538 ;  Pliocene  of, 
520;  Silurian  of,  388  ;  Triassic  of,  447. 

Eurylepis,  426. 

Eurynotus,  426. 

Eurypterida,  381,  39if,  402,  424. 

Eurypterus,  391,  402. 

Eutaw  stage,  475. 

Exogyra,  465,  488. 

FACIES,  388. 

Falkland  Islands,  Devonian,  399. 

False-bedding,  224. 

False  Coal  Measures,  411. 

Fan,  alluvial,  137*.  138. 

Fan  fold,  240. 

Fasciolaria,  521*. 

Fault,  63,  243f ;  diminution  of,  259 ;  dip 

of,  243. 

Fault  block,  248,  337. 
Fault  rock,  246. 
Fault  scarp,  248*. 
Faulted  inlier,  320 ;  outlier,  320. 
Faults,  compound,  247;  cross,  248;  dip, 

246,  250,  251*;    normal,  244*,  245*; 

reversed,  252*,  253*;  step,  248,  250*; 

strike,  246,  249*;    thrust,  252*,  253*; 

trough,  248. 

Fauna,  367 ;  geographical,  348. 
Favistella,  380*. 


INDEX 


559 


Favosites,  389,  392*. 

Felsite,  igSf,  297. 

Felsitic  texture,  igof,  196. 

Felspar,  i6f,  190,  193,  194,  196,  289,  293, 
294  ;  weathering  of,  74. 

Felspar  porphyry,  198. 

Felspathic  sandstone,  206. 

Felspathoids,  17^,  193,  194. 

Ferns,  Carboniferous,  418  ;  Cretaceous, 
485;  Devonian,  400;  Eocene,  502;  Ju- 
rassic, 462;  Mesozoic,  441 ;  Palaeozoic, 
367;  Permian,  432;  Triassic,  448. 

Ferro-magnesian  minerals,  193,  196. 

Figs,  fossil,  516. 

Fire-clay,  I35t,  2o8f,  413. 

Fishes,  Carboniferous,  426;  Cretaceous, 
490;  Devonian, 404,  406;  Eocene,  503; 
Jurassic,  467 ;  Oligocene,  508 ;  Ordo- 
vician,  384;  Permian,  435;  Silurian, 
393 ;  Triassic,  452. 

Fissility,  26o*t,  334 ;  cause  of,  262. 

Fissure,  243  ;  earthquake,  63  ;  eruptions, 
56,519;  springs,  93*. 

Fjords,  307*,  3iif. 

Flabellaria,  502*. 

Flagstone,  206. 

Flamingoes,  fossil,  516. 

Flies,  fossil,  442,  465. 

Flightless  birds,  503. 

Flint,  i6f,  210,  214. 

Flint  conglomerate,  207. 

Flood  plain,  137. 

Flora,  367 ;  Glossopteris,  437. 

Florida,  anticline,  520;  island,  499;  pe- 
ninsula, 514. 

Florissant,  Oligocene  lake,  507. 

Fluor-spar,  24. 

Fluviatile  deposits,  136. 

Focus  of  earthquake,  62. 

Folded  strata,  318. 

Folding,  causes  of,  254 ;  experiments  on, 
256. 

Folds,  232f,  233,  237*,  238. 

Foliated  rocks,  295. 

Foliation,  26of,  290. 

Foot  wall,  243. 

Foraminifera,  Carboniferous,  421 ;  Creta- 
ceous, 487;  Devonian,  400;  Eocene, 
502;  Jurassic,  462 ;  Ordovician,  379. 

Foraminiferal  ooze,  176,  178*. 

Forests,  buried,  67. 


Fort  Pierre  substage,  475,  482^-. 

Fossils,  343  ;  in  metamorphic  rocks,  287  ; 
modes  of  preservation  of,  345. 

Fox  Hills  substage,  475,  482^ 

Fragmental  products  (volcanic),  5of, 
277t. 

Fragmental  texture,  190. 

Fredericksburg  stage,  475. 

Fresh-water  lakes,  deposits  in,  143 ;  lime- 
stone, 212. 

Front  Range  (of  Rocky  Mountains),  397. 

Frost,  destructive  work  of,  80. 

Fusulina,  421,  423* ;  limestone,  417. 

Fusus,  488. 

GABBRO,  202f,  284. 

Galena,  268. 

Gallinaceous  birds,  fossil,  516. 

Gangamopteris,  437. 

Ganges,  delta  of,  141,  142;  material 
transported  by,  102. 

Gangue,  266. 

Gannett,  H.,  322. 

Gannister,  135. 

Ganodonts,  505. 

Ganoidei,  406,  426,  453,  467,  490. 

Garnet,  289. 

Gaseous  products  (volcanic),  53. 

Gastropoda,  Cambrian,  374;  Carbonif- 
erous, 425 ;  Cretaceous,  488 ;  De- 
vonian, 402;  Eocene,  503;  Jurassic, 
466 ;  Mesozoic,  442 ;  Ordovician,  382 ; 
Permian,  434;  Silurian,  393;  Triassic, 
452. 

Geanticline,  236. 

Geikie,  Sir  Archibald,  366. 

Geikie,  J.,  526,  535,  536. 

Geographical  changes,  shown  by  fossils, 
352;  by  rocks,  220;  faunas,  348. 

Geology,  defined,  i;  dynamical,  7,  27f; 
historical,  7,  343t;  history  of,  1-4; 
physical,  7;  physiographical,  7,  soof; 
structural,  7,  i86f;  subdivisions  of,  7. 

Georgian  epoch,  355,  368. 

Geosyncline,  236. 

Geyserite,  128,  210. 

Geysers,  96. 

Giant's  Causeway,  262. 

Giant  Spring,  95. 

Gingko,  432,  502. 

Giraffes,  fossil,  522. 


56o 


INDEX 


Glacial,  deposits,  153;  drift,  311,  527; 
epoch,  525  ;  causes  of,  537  ;  effects  on 
topography,  534;  lakes,  311 ;  Permian, 
438;  series,  529;  stages,  526;  striae, 
in*,  112;  valleys,  310. 

Glaciers,  104 ;  denudation  by,  310;  ero- 
sion, no;  flow  of,  107;  formation  of, 
105;  transportation  by,  113;  troughs, 
112;  varieties  of,  no. 

Glass,  volcanic,  46. 

Glassy  texture,  iSgf,  196. 

Glauconite,  22f,  214. 

Glauconitic  Beds,  475. 

Globigerina,  176,  487  ;  ooze,  176. 

Glossopteris,  437  ;  ilora,  437. 

Glyptodon,  538. 

Glyptostrobus,  504*. 

-Gneiss,  296;  Archaean,  358. 

Gold,  266;  deposition,  130. 

Gomphoceras,  401*,  402. 

Gondwana  series,  437!,  448,  461. 

Goniatites,  401*,  403,  419*,  426,  434. 

Goodnight  stage,  496,  5i8f. 

Gordon,  C.  H.,  296. 

Granatocrinus,  422. 

Grand  Canon  of  the  Colorado,  100. 

Granite,  igSf,  284,  296;  Archaean,  358; 
binary,  198  ;  conglomerate,  207 ;  dis- 
integration of,  74 ;  exfoliation  of,  84*  ; 
family,  196;  gneissoid,  358;  joints  of, 
263  ;  porphyry,  198  ;  soda-,  198. 

Granitic  gneiss,  296. 

Granitite,  198. 

Granitoid  texture,  i9of,  196. 

Graphite,  295f,  298 ;  schist,  298. 

Graptolites,  Cambrian,  371 ;  Devonian, 
400;  Ordovician,  379;  Silurian,  389. 

Graptolithes,  392*. 

Grass,  protection  of  rocks  by,  122. 

Grasses,  Eocene,  502;  Miocene,  515. 

Grasshoppers,  fossil,  465. 

Gravel,  207;  river,  139. 

Gravity  fault,  244*,  245*,  246^ 

Great  Basin,  309 ;  land,  440. 

Great  Britain,  Algonkian  of,  363. 

Great  Dismal  Swamp,  133,  134*,  414. 

Great  Lakes,  origin  of,  534. 

Great  Plains,  glaciation  of,  527. 

Great  sea  wave,  62. 

Greece,  earthquake  of  1870,  63. 

Green  mud,  175. 


Green  River,  325. 

Green  River  Shales,  505 ;  substage,  496. 

Greensand,  i75t,  214. 

Greenland,  Carboniferous  of,  417 ;  de- 
pression of,  67 ;  Devonian  of,  399 ; 
Eocene  flora  of,  502 ;  ice-sheet  of,  108. 

Greenstone,  200. 

Greywacke,  293f,  368 ;  slate,  294. 

Griffith  ides,  424. 

Ground  ice,  115. 

Group  (of  strata),  354. 

Gryph&a,  463*,  465,  488. 

Guano,  130. 

Guard  (of  Belemnite) ,  467. 

Gulls,  fossil,  503,  516. 

Gymnospermae,  400;  Palaeozoic,  367; 
Permian,  432. 

Gypsum,   23^,  209,  429;   deposition  of, 

151- 
Gyroceras,  434. 

HADE  (of  fault) ,  243. 

Hadrosaurus,  491. 

Haematite,  24;  brown,  24. 

Hairy  elephant,  538  ;  rhinoceros,  538. 

Hale-mau-mau,  40*.  42*. 

Halobia,  451. 

Halysitcs,  389. 

Hamilton  epoch,  355 ;  series,  394,  396! ; 

stage,  394. 
Hanging  wall,  243. 
Hardness  of  minerals,  13. 
Harpoceras,  467. 
Heave  of  fault,  244. 
Heavy  spar,  266. 
Heligoland,  destruction  of,  117. 
Heliolites,  389. 
He  Hop  by  Hum,  400,  401*. 
Helix,  504*. 

Hell  Gate,  tidal  race,  119. 
Hemiaspis,  391. 
Henry  Mountains,  284,  338. 
Heptodon,  505. 
Herculaneum,  38,  51. 
Herring,  fossil,  490. 
Hesperornis,  492. 
Hexacoralla,  441,  450,  463. 
Hexagonal  system  of  crystals,  10. 
Hickories,  fossil,  515. 
Highlands  of  the  Hudson,  333 ;  of  New 

Jersey,  333. 


INDEX 


56l 


High  Plateaus  of  Utah,  316;  origin  of, 
483;  uplifts,  534. 

Hillside  springs,  92*,  93. 

Himalaya  Mountains,  325,  496  ;  origin  of, 
515  ;  Pleistocene  glaciers  of,  536;  rain- 
fall of,  78. 

Hippopotamus,  fossil,  522,  538. 

Hippotherium ,  517. 

Historical  geology,  7,  343t. 

History,  human  and  geological  com- 
pared, 350. 

Hoang-ho  River,  delta  of,  142. 

"  Hog-backs,"  315*,  318. 

Holaster,  487. 

Holoptychius,  406*. 

Holostomata,  466. 

Holothuroidea,  424. 

Homalonotus,  400,  401*. 

Honduras,  Triassic  of,  447. 

Hoplites,  488. 

Horizontal  and  oblique  bedding,  273. 

Hornblende,  2of,  193,  198,  199,  202,  289, 
29°.  358I  andesite,  200;  gneiss,  296; 
granite,  198  ;  schist,  297. 

Hornblendite,  202. 

Hornfels,  289. 

Hornstone,  210,  289. 

"  Horses  "  in  coal  seams,  273. 

Horses,  fossil,  505,  506,  509,  517,  522,  524, 
538. 

Horse-shoe  crabs,  fossil,  391,  465. 

Horsetails,  Carboniferous,  420;  Creta- 
ceous, 485  ;  Eocene,  502 ;  Jurassic,  462  ; 
Palaeozoic,  367  ;  Triassic,  448,  449. 

Horsetown  stage,  475,  478f. 

Hot-spring  deposits,  130. 

Hudson  River,  submarine  channel  of, 
68. 

Hudson  stage,  375,  376f. 

Huerfano  Canon,  499. 

Humous  acids,  their  effects  in  decompos- 
ing rocks,  75. 

Huronian  period,  362. 

Hyaenas,  fossil,  522,  538. 

Hy&nodon,  508. 

Hyalite,  15. 

Hydration  of  minerals,  73. 

Hydraulic  limestone,  213. 

Hydroid  Corals,  389. 

Hydrozoa,  371. 

Hyperodapedon,  454. 

2  O 


Hypersthene,  20. 
Hypsocormus,  468*. 
Hypothesis,  Nebular,  356. 
Hypotheses,  uses  of,  5. 
Hyracotherium,  505. 
Hyracodon,  509,  510*. 
Hystricomorpha,  523. 

IBIS,  fossil,  503,  516. 

Ice,  coast,  115;  ground,  115. 

Ice-sheet,   Antarctic,   108 ;    Cordilleran, 

526 ;  Greenland,  108 ;  Laurentide,  526 ; 

Keewatin,  526;    Pleistocene   deposits, 

527- 

Icebergs,  116;  deposits  by,  158. 
Iceland,  96. 
Iceland  spar,  22. 
Ichthyornis,  492. 
Ichthyosauria,  454,  469,  490. 
Ichthyosaurus,  469*. 
Igneous  agencies,  34,  6g\. 
Igneous  rocks,  188,  445;   weathering  of, 

74 ;  veins,  279. 
Iguanodon,  491. 
Illcznus,  381,  391. 
Illinois  stage,  529,  530. 
Ilmenite,  25!,  193. 
Inarticulata,  373. 

Inclined  folds,  239,  241*;  strata,  314. 
Indian  swallows,  fossil,  516. 
Indiana-Illinois  coal  field,  415. 
Infusorial  earth,  214. 
Inherited  drainage,  326. 
Initial  dip,  231,  256,  257*. 
Injection,  289. 
Inlier,  319;  faulted,  320. 
Inoceramus,  488,  489*. 
Interior  sea  of  North  America,  375,  386, 

409,410,411. 
Intratelluric  crystals,  190. 
Intrusive  rocks,  277;   sheets,  50,  280*, 

281*.  282. 

Inverted  folds,  239. 
lone  stage,  512. 
Iowa-Missouri  coal  field,  415. 
lowan  stage,  529,  530. 
Irish  deer,  538. 
Iron,  colouring  effects  of,  25;   deposits 

of,  130;  minerals,  24 ;  oxides,  solution 

of,  75  ;  pre-Cambrian,  364. 
Iron-ore  stage,  475. 


562 


INDEX 


Iron  pyrites,  25. 
Ironstone,  211. 
Irregulares,  464^  487,  502. 
Irwell  River,  terraces  of,  140. 
Isastrcea,  463. 
Ischypterus,  453. 
Islands,  volcanic,  55,  56. 
Isoclinal  folds,  240,  241*. 
Isometric  system  of  crystals,  9. 
Isopoda,  402,  465. 
Isostasy,  68,  69. 
Isotropic  crystals,  u. 

JACKSON  STAGE,  496. 

James  River  stage,  475. 

Japan,  earthquakes  of,  64;   Triassic  of, 

448. 

Jelly-fish,  Cambrian,  371. 
John  Day  stage,  496,  5i2f. 
Johnstown,  Pa.,  flood  of  1889,  101. 
Joints,  48,  81,  262f. 
Jupiter  Serapis,  temple  of,  66. 
Jura  Mountains,  323. 
Jurassic  period,  457. 

KAMES,  312,  528t. 

Kansan  stage,  529,  530. 

Kaolin,  90,  2O7f ;  formation  of,  74. 

Kaolinite,  22. 

Karoo  series,  437,  448. 

Keewatin  glacier,  526. 

Kemp,  J.  F.,  193,  194,  296. 

Keokuk  substage,  409. 

Kettle  moraine,  155. 

Keuper,  447. 

Kilauea,  40*,  41*,  42*,  44*,  45*. 

Kinderhook  stage,  409,  411. 

Kittatinny    Mountain,  314;     peneplain, 

342;  plain,  481. 
Knoxville  stage,  475,  478f. 
Koninckina,  451. 
Kootanie  stage,  475,  477^ 
Krakatoa,  eruption  of,  38*. 

LABRADORITE,  16,  iTf,  201,  202. 
Labyrinthodon,  453. 
Laccolite,  283. 

Laccolith,  283*.  284*,  285*.  338. 
Laccolithic  mountains,  338. 
Lacertilia,  469,  491,  495. 


Lcelaps,  492. 

Lafayette  formation,  519,  530. 

Lake,  Agassiz,  531 ;  Bonneville,  146, 
147*.  330,  533 ;  Erie,  535 ;  Great  Salt, 
146, 147* ;  Huron,  535  ;  Iroquois,  535  ; 
Lahontan,  151,  533;  Michigan,  535; 
Mono,  151 ;  Ontario,  535 ;  Pyramid, 
151 ;  Superior,  deposits  in,  145. 

Lakes,  119;  deposits  in,  143;  erosion 
by,  120;  fossils  in,  344;  fresh-water, 
143;  glacial,  311;  salt,  148;  shore- 
lines of,  121 ;  soda,  152. 

Laminae,  219. 

Land  barriers,  353 ;  bridges,  353 ;  Eo- 
cene, 501 ;  Pleistocene,  538. 

Landslips,  92. 

Lapilli,  51. 

Lapworth,  Professor,  375. 

Laramie  stage,  475,  482f. 

Lateral  compression,  255,  336;  causes 
of,  337 ;  effects  of,  265,  298,  336. 

Laurels,  fossil,  502,  516. 

Laurentian    lakes,    534 ;     deposits    in, 

145. 

Laurentide  glacier,  526. 
Lava,  42 ;  bedding  of,  47 ;   composition 

of,  44 ;  cooling  of,  46 ;  flows  or  sheets, 

276*,  277 ;  motion  of,  44 ;    stalactites, 

45* ;  texture  of,  46. 
Layer,  219. 
Lead  deposits,  267. 
Lemming,  fossil,  538. 
Lemuroidea,  505,  506,  508. 
Lepadocrinus \  392*. 
Leperditia,  383*. 
Lepidodendrids,  400. 
Lepidodetidron,  420,  423*,  432. 
Lepidolite,  19. 
Lepidoptera,  465. 
Lepidotus,  453,  468. 
Leptana,  382. 
Leptolepis,  468. 
Leptotragulus,  506. 
Leucite,  i8f,  199,  201 ;  basalt,  201. 
Level,  changes  of,  64,. 
Liassic  series,  457,  458,  460. 
Lie has ;  390*,  391,  400. 
Lignite,  2i5f,  507. 
Lignitic  stage,  496. 
Lima,  434. 
Limburgite,  202. 


INDEX 


563 


Limestone,  21  if,  292;  banks,  172;  con- 
glomerate, 207 ;  coral,  169*,  213 ;  cri- 
noidal,  213;  fresh-water,  213;  hy- 
draulic, 213;  magnesian,  213;  shell, 
171,  213;  weathering  of,  76. 

Limonite,  24. 

Limuloidea,  391,465. 

Lirnuhis,  465. 

Lingulella,  373*,  392*. 

Lions,  fossil,  538. 

Liparite,  198. 

Liriodendron,  485*. 

Lithodomus,  66. 

Lithostrotion,  419*,  422. 

Litopterna,  523. 

Little  Sun  Dance  Hill,  283,  284*,  338. 

Littoral  deposits,  162. 

Lituites,  384,  393. 

Live  oaks,  fossil,  516. 

Livingstone  stage,  475,  483^ 

Lizards,  fossil,  469,  491,  495,  503. 

Llamas,  fossil,  517,  522,  524,  538. 

Loess,  I25f,  530. 

Longitudinal,  streams,  317,  318 ;  valleys, 

Si?- 

Lookout  Mountain,  332. 

Lophophyllum,  422,  423*. 

Loup  Fork  stage,  496,  5i3f. 

Loup  River,  322. 

Lower,  Barren  stage,  409  ;  Carboniferous 
epoch,  355  ;  Carboniferous  series,  409  ; 
Claiborne  stage,  496;  H el derberg  ep- 
och, 355  ;  Helderberg  series,  385,  387  ; 
Pentamerus  stage,  385;  Productive 
stage,  409. 

Loxonema,  423*,  425,  452. 

Lycopodiaceae,  Carboniferous,  418  ;  De- 
vonian, 400;  Permian,  432;  Triassic, 
449. 

Lyell,  Sir  Charles,  78,  98,  497. 

Lytoceras,  466. 

MACROTVENIOPTERIS,  449. 
Macrura,  464. 

Magma,  191 ;  crystallization  of,  192. 
Magnesian  limestone,  2i3f,  294. 
Magnetite,  24t,  193,  194,  198,  201,  289. 
Magnolias,  fossil,  502,  515,  516,  521. 
Malaspina  Glacier,  108*,  no,  156*. 
Mallet,  58. 
Malm,  457,  459. 


Mammalia,  Cenozoic,  495 ;  Cretaceous, 
493;  Eocene,  503,  505,  506;  Jurassic, 
473;  Miocene,  516;  Pleistocene,  538 ; 
Pliocene,  521 ;  Tertiary  of  South  Amer- 
ica, 523 ;  Triassic,  456. 

Mammoth,  538;  Cave,  89;  Hot  Springs, 
127*,  128*. 

Man,  appearance  of,  539. 

Manasquan  stage,  475. 

Maples,  fossil,  486,  502,  515,  516. 

Marattiacecs,  418,  448. 

Marcasite,  25. 

Marcellus  stage,  394. 

Margarita,  489. 

Marginella,  521. 

Marine  deposits,  160. 

Marl,  208. 

Marmots,  fossil,  509. 

Marshall  Beds,  411. 

Marsupialia,  523,  539. 

Marsupites,  487,  489*. 

Massive  rocks,  189,  218,  274t. 

Master  joints,  263. 

Mastodon,  516,  517,  522,  524,  538. 

Mastodonsaurus,  453. 

Matawan  stage,  475. 

Mato  Tepee,  262,  283,  285*. 

Maturity,  of  rivers,  321,  322,  329;  of  to- 
pography, 302. 

Mauch  Chunk  stage,  409,  4iof. 

Mauna  Loa,  40,  46,  53*,  54. 

Mechanical  deposits,  205. 

Mecklenburgian  stage,  536. 

Medina  stage,  385,  386f. 

Mediterranean  region,  earthquakes  of, 
64. 

Medlicottia,  433*,  434. 

Meekoceras,  452. 

Megaceros,  538. 

Megalonyx,  538. 

Megalosaurus,  471,  491. 

Megalurus,  468. 

Megatherium,  538. 

Melonites,  419*,  424. 

Menaspis,  435. 

Meniscoessus,  493. 

Merced  series,  496,  5i8f. 

Mersey  River,  terraces  of,  140. 

Mesa,  80,  313. 

Mesohippus,  509*. 

Mesozoic  era,  355,  44if. 


INDEX 


Metals,  native,  266. 

Metalliferous  veins,  266. 

Metamorphic  rocks,  188,  217,  293f;  Al- 
gonkian,  363  ;  Archaean,  358  ;  foliated, 
295;  fossils  in,  287;  non-foliated,  293; 
schistose,  295. 

Metamorphism,  287 ;  causes  of,  291 ;  con- 
tact, 288 ;  dynamic,  291, 335  ;  regional, 
290;  thermal,  291. 

Metamynodon,  509. 

Mexican  onyx,  209. 

Mica,  i8f,  194,  289,  293,  294,  358 ;  schist, 
289,  297f;  syenite,  199. 

Micaceous  sandstone,  206. 

Mice,  fossil,  509. 

Michigan  coal  field,  415. 

Micrabacia,  489*. 

Microconodon,  456. 

Microlestes,  456. 

Midway  stage,  496. 

Migrations,  Cenozoic,  495;  Pleistocene, 

537- 

Millstone  Grit  stage,  409,  4i2f. 

Mineral,  9  ;  springs,  95  ;  veins,  247,  265^ 

Mineralizers,  igif,  292. 

Minerals,  accessory,  193;  essential,  193; 
original,  194;  rock-forming,  8,  14; 
secondary,  194. 

Miocene  beds,  tilting  of,  519. 

Miocene  epoch,  355,  5iif;  series,  496. 

Missionary  Ridge,  332. 

Mississippi  River,  325  ;  delta  of,  141, 142; 
materials  carried  by,  102. 

Mississippi  valley,  Carboniferous  of,  409, 
411;  earthquakes  of,  61,  64;  Pleisto- 
cene succession  in,  529. 

Mississippian  series,  409,  41  if. 

Mitra,  516,  521*. 

Mollusca,  Cambrian,  374;  Carbonifer- 
ous, 425;  Cretaceous,  488;  Devo- 
nian, 402 ;  Jurassic,  465 ;  Mesozoic, 
442 ;  Ordovician,  382 ;  Palaeozoic,  367 ; 
Permian,  434 ;  Silurian,  393 ;  Triassic, 

45i. 

Molluscan  deposits,  modern,  171. 
Monchiquites,  202. 
Monmouth  stage,  475. 
Monkeys,  fossil,  505,  508,  522,  523. 
Mono  Lake,  151. 
Monoclinal  fold,  241*,  242. 
Monoclinic  system  of  crystals,  10. 


Monoclonius,  492. 

Monocotyledons,  441,  462,  486. 

Monometric  system  of  crystals,  9. 

Monotis,  451*. 

Monotremata,  493. 

Montana  stage,  475,  48if. 

Monte  Diablo  range,  520. 

Monte  Somma,  38,  39,  55*. 

Montlivaultia,  463. 

Monument  Park,  80. 

Moraine,  113;  ground,  114,  527;  kettle, 

155, 311 ;  lateral,  113,  155  ;  medial,  113  ; 

terminal,  114,  154,  311,  531. 
Morainic  plains,  528. 
Mosasauria,  490. 
Moulds,  346. 
Mt.  Hood,  55 ;  Rainier,  55,  275 ;  Shasta, 

54*.  55.  275- 
Mt.  Vernon  stage,  475. 
Mountain,  332;  chain,  333;  range,  332  ; 

system,  333. 

Mountain  Limestone,  416. 
Mountains,  date  of,  338 ;  denudation  of, 

339 ;    laccolithic,  338  ;    synclinal,  340 ; 

table,  332. 
Mud,  174;   blue,  175;   green,  175;  red, 

175 ;  volcanic,  176. 
Mudstone,  2o8f,  293. 
Mullet,  fossil,  490. 
Multituberculata,  493t,  505. 
Murchison,   Sir   R.,  368,  375,  385,  394, 

428. 

Murchlsonia,  382,  383*,  452. 
Murex,  488,  516,  521*. 
Murray  &  Renard,  161. 
Muschelkalk,  447. 
Muscovite,  igf,  198 ;  granite,  198. 
Musk-ox,  fossil,  538. 
Mustelines,  522. 
Myallna,  433*,  434. 
Mylodon,  538. 
Myodes,  538. 
Myophoria,  451*. 
Myriapoda,  424. 
Myrtles,  fossil,  502,  516. 

NASSA,  521. 
Natica,  521. 
Native  metals,  266. 

Natural  Bridge,  Arizona,  90;   Virginia, 
90,  91*. 


INDEX 


565 


Nautiloidea,  382;  Carboniferous,  425; 
Cretaceous,  490;  Devonian,  402 ;  Eo- 
cene, 503;  Jurassic,  466;  Ordovician, 
382  ;  Silurian,  393  ;  Triassic,  452. 

Nautilus,  374,  433*.  434,  490. 

Nebraska  substage,  496,  513!- 

Nebular  Hypothesis,  356. 

Neck,  volcanic,  274,  275*. 

Neocene,  497. 

Neocomian,  484. 

Neocrinoidea,  441,  450,  464. 

Neogene,  497. 

Nepheline,  i8f,  199,  201 ;  basalt,  201. 

Nerinia,  466. 

Neumayr,  M.,  461. 

Neuroptera,  402,  424,  442,  465. 

Neuropteris,  389,  432. 

Neve,  106. 

Newark  series,  443, 445-}-. 

New  Scotland,  Permian  of,  431. 

New  Zealand,  Cretaceous  of,  485  ;  Eocene 
of,  501;  geysers  in,  96;  Miocene  of, 

5i5- 
New  Zealand  Alps,  Pleistocene  glaciers, 

536. 

Niagara  epoch,  355;  River,  98,  100;  se- 
ries, 385  ;  stage,  385,  387^ 

Nile,  delta  of,  142. 

Niobrara  substage,  475,  48if. 

Nodosaria,  489*. 

Nodules,  227,  229*. 

Norite,  202. 

Normal  fault,  244*,  245*,  246f;  cause  of, 

258- 

North  America,  Algonkian  of,  362;  Ar- 
chaean, 359;  Cambrian,  369  ;  Carbonif- 
erous, 409;  Cretaceous,  474;  Devo- 
nian, 394 ;  Eocene,  497  ;  Jurassic,  457  ; 
Miocene,  511;  Oligocene,  506 ;  Ordo- 
vician, 375  ;  Permian,  428 ;  Pleistocene, 
526;  Pliocene,  518 ;  Silurian,  385;  Tri- 
assic, 444. 

North  Carolina  sounds,  182. 

Notkosaurus,  454. 

Nova  Zembla,  Carboniferous  of,  417, 421 ; 
Jurassic  of,  461. 

Novaculite,  206. 

Nullipores  in  coral  reefs,  167. 

Nummulites,  500,  502. 

Nunataks,  108,  109*. 


OAHU,  56. 

Oaks,  fossil,  486,  502,  515. 

Oblique  bedding,  231,  273;  system,  10. 

Obsidian,  46-)%  I97f. 

Obsidian  Cliff,  49*.  192,  262. 

Octahedron,  9*,  10*. 

Oilstone,  206. 

Old  age,  of  rivers,  321,  323 ;  of  topogra- 
phy, 302. 

Old  Red  Sandstone,  398. 

Olenellus,  372 ;  Fauna,  368. 

Olenoides,  373*. 

Olenus  Fauna,  368. 

Oligocene  epoch,  355,  5o6f;  series,  496. 

Oligoclase,  i6f,  17,  196. 

Oligoporus,  424. 

Oliva,  504*. 

Olivine,  2of,  22, 193, 201, 202, 295 ;  basalt, 
201. 

Omosaurus,  471. 

Oneida  substage,  386. 

Onondaga  epoch,  355 ;  series,  385. 

Onychocrinus,  419*,  424. 

Onyx  marble,  209. 

Oolite,  169,  209f. 

Oolitic  series,  457. 

Ooze,  diatom,  179,  180*;  foraminiferal, 
176,  178* ;  Globigerina,  176 ;  pteropod, 
178,  179*;  radiolarian,  179;  siliceous, 
214. 

Opal,  15. 

Open  folds,  237*,  239. 

Ophicalcites,  295. 

Ophioglossacece,  418. 

Ophiuroidea,  381. 

Orbitolites,  502. 

Ordovician  period,  355,  375f. 

Oreodonts,  506,  509. 

Organic,  accumulations,  211 ;  agencies, 
122. 

Original  minerals,  194. 

Oriskany  epoch,  355  ;  series,  394,  395f. 

Ornithomimus,  492. 

Ornithopsis,  491. 

Ornithostoma,  491. 

Orogenic  diastrophism,  301. 

Orthis,  382,  383*,  392*.  402,  425. 

Orthisina,  382. 

Orthoceras,  383*,  384,  392*,  393,  402,  425, 
434.  452. 

Orthoclase,  i6f,  196,  198,  199,  291,  296. 


566 


INDEX 


Orthoptera,  402,  424,  442,  465. 
Orthorhombic  system  of  crystals,  10. 
Osage  stage,  409,  411. 
Ostracoda,  373,  382,  424. 
Ostracoderms,  Devonian,  403;   Ordovi- 

cian,  384;  Silurian,  393. 
Ostrea,  465,  488,  489*,  504*. 
Otozamites.  449,  451*. 
Otters,  fossil,  517. 
Ouachita  Mountains,  333,  334,  439. 
Outcrop,  233. 
Outlier,  319 ;  faulted,  320. 
Overlap,  271*. 
Overturned  folds,  237*,  239. 
Overwash  plain,  312,  528. 
Owen's  Valley,  earthquake  of,  63. 
Owls,  fossil,  503,  516. 
Oxen,  fossil,  522. 
Oxidation  of  minerals,  73. 
Oxycena,  505. 

PACHY^NA,  505. 

Pacific  coast,  Pleistocene  submergence, 

533- 

Palseocrinoidea,  424,  441,  450. 
Palaeoechinoidea,  441,  450. 
Palaeogene,  497. 
Palceohatteria,  435. 
Palcsoniscus,  426. 
Palceoayops,  506. 
Palceozamia,  462. 
Palaeozoic  era,  355,  365^ 
Palisades  of  Hudson,  82,  280,  281*,  282*. 
Palms,  fossil,  486,  502,  508,  516. 
Palustrine  deposits,  132. 
Pampas,  loess  of,  125. 
Panthers,  fossil,  517. 
Paradoxides,  372 ;  Fauna,  368. 
Paramorphic  minerals,  290. 
Parrots,  fossil,  516. 
Partings  (of  coal  seams),  412. 
Patagium,  472. 
Patagonia,  glaciation  of,  537. 
Patagonian  series  or  stage,  523. 
Peat,  i33t,  215. 
Peat  bogs,  133,  134,  135 ;  theory  of  coal 

formation,  213. 

Pebbles,  river,  137 ;  wind  cut,  86. 
Peccaries,  fossil,  509,  522,  538.. 
Pecopteris,  423*,  432. 
Pecten,  451,  504*. 


Pedlomys,  493. 
'elagic  deposits,  176. 
'elicans,  fossil,  503,  516. 

Pelycypoda,  see  Bivalves. 

Peneplain,  3iof,  319,  323,  341 ;    Kitta- 
tinny,  342 ;  Shenandoah,  342. 

Pentacrinus,  463*,  464. 

Pentameridce,  391. 

Pentamerus,  393,  425. 

Pentremites,  419*,  422. 

Perchasrus,  509. 

Peridotite,  202. 

Period,  geological,  354. 

Perisphinctites,  467. 

Perissodactyla,  505,  506,  509. 

Perlite,  197. 

Permian  period,  355,  428f. 

Permo-Carboniferous,  436. 

Petrifaction,  346,  347. 

Petrography,  194. 

Phacops,  391,  400,  401*. 

Phascolotherium,  473. 

Phenocrysts,  190. 

Phillipsastrcea,  400. 

Phillipsia,  419*,  424,  432. 

Pholadomya,  466. 

Phonolite,  iggt,  284. 

Phosphate  deposits,  130. 

Phragmoceras,  392*,  393,  402. 

Phragmocone  (of  Belemnites) ,  467. 

Phyllite,  294. 

Phylloceras,  466. 

Phyllograptus,  383*. 

Phyllopoda,  373,  382,  424. 

Phyllotheca,  437. 

Piedmont  glaciers,  no. 

Pinacoceras,  452. 

Pines,  fossil,  502. 

Pinites,  462. 

Pipes,  89. 

Pirsson,  L.  V.,  202. 

Pisolite,  209. 

Pitchstone,  197. 

Placenticeras,  488. 
Plagioclase,  I7f,  198,  202,  358. 
Plain,  of  marine  denudation,  304;  over- 
wash,  312,  528 ;   of  subaerial  denuda- 
tion, 310,  319,  323. 

Planorbis,  504*. 

Plants,  Cambrian,  371 ;    Carboniferous, 
418  ;  Cenozoic,  495  ;  Cretaceous,  485 ; 


INDEX 


567 


Devonian,  399 ;  Eocene,  501 ;  Jurassic, 
462;  Mesozoic,  441 ;  Miocene,  515; 
Oligocene,  508 ;  Ordovician,  379 ; 
Palaeozoic,  367;  Permian,  432,  437; 
Pleistocene,  537 ;  Pliocene,  521 ;  Silu- 
rian, 389;  Triassic,  448. 

Plateau,  Pourtales,  173. 

Plateaus  of  Arizona,  316;  of  Utah,  316. 

Platinum,  266. 

Platte  River,  322. 

Platyceras,  419*.  425. 

Platycrinus,  400,  424. 

Platyostoma,  392*,  393. 

Platystroph'ia,  382. 

Pleistocene  epoch,  355,  525^  529. 

Plesiosauria,  454,  470,  490,  495. 

Plesiosaurus,  470*. 

Pleuracanthus,  426,  434*. 

Pleurotomaria,  382,  423*,  425,  466. 

Pliauchenia,  517. 

Plication,  240. 

Pliocene  epoch,  355,  496,  5i8f. 

Pliosaurus,  470. 

Plutonic  rocks,  iSgf,  277^ 

Po,  delta  of,  142. 

Pocket  gophers,  fossil,  509. 

Pocono  stage,  409,  410. 

Poebrotherium,  509. 

Polymastodon,  505. 

Polysynthetic  twinning,  14. 

Pompeii,  38,  51,  52*. 

Ponderosa  Marls,  475. 

Popanoceras,  434. 

Poplars,  fossil,  486,  502,  515,  516. 

Populus,  485. 

Porcelain  clay,  207. 

Porphyritic  texture,  47,  I9of,  196. 

Portage  stage,  394. 

Portheus,  490. 

Pot  holes,  86,  330. 

Potomac  River,  324,  327,  328;  series, 
474t,  475- 

Potsdam  epoch,  355,  36?-. 

Potter's  clay,  207. 

Pourtales  plateau,  173. 

Pre-Cambrian  ores,  364;  periods,  356; 
rocks,  364. 

Precipitates,  chemical,  208. 

Prehistoric  time,  356. 

Prestwich,  Sir  J.,  59. 

Primary  rocks,  495. 


Prisms,  9*,  10*. 

Procamelus,  517. 

Productidce,  391,  434. 

Produdus,  402,  419*,  423*,  425,  451. 

Proetus,  391,  424. 

Proganosauria,  435. 

Protapirus,  509. 

Proterosaurus,  435. 

Protoceras,  509. 

Protohippus,  517. 

Protoreodon,  506. 

Pseudodiadema,  487,  489*. 

Pseudomorphs,  fossil,  346 ;  mineral,  13. 

Pteraspis,  403. 

Pterichthys,  404*. 

Pterinea,  401*.  402. 

Pterodon,  508. 

Pterophyllum,  449. 

Pteropod  ooze,  178,  179*. 

Pteropoda,  374,  393. 

Pterosauria,  47  if,  491,  495. 

Pterygotus,  391,  402. 

Ptilodus,  493,  505. 

Ptychites,  434. 

Ptychoceras,  489*,  490. 

Ptyonius,  427. 

Purbeck  stage,  493. 

Puerco  stage,  496,  498,  505. 

Pumice,  197. 

Pumiceous  texture,  189. 

Purpurina,  466. 

Pyramid,  10*. 

Pyramid  Lake,  151. 

Pyramidal  system  of  crystals,  9. 

Pyrenees,  515. 

Pyrite,  25  ;  auriferous,  266 ;  deposition  of, 

130. 

Pyroclastic  products,  277  ;  rocks,  203. 
Pyrotherium  beds,  523. 
Pyroxene,  igf,  193,  194,  199,  202,  289; 

andesite,  200. 
Pyroxenite,  202. 
Pythonomorpha,  490,  495. 

QUAIL,  fossil,  503. 

Quartz,  I5f,  193,  194,  196,  198,  201,  266, 
289,  291,  296,  358;  deposition  of,  130; 
conglomerate,  207 ;  diorite,  200 ;  por- 
phyry, I98f,  279 ;  schist,  297 ;  smoky, 
15 ;  trachyte,  198. 

Quartzite,  289,  293^ 


568 


INDEX 


Quaternary  period,  355,  495,  5251. 
Quatlambabergen,  437. 
Quenstedioceras,  463*. 
Quicklime,  292. 

RABBITS,  fossil,  509. 

Radiolaria,  364;  Jurassic,  462;  Ordovi- 
cian,  379. 

Radiolarian  ooze,  179. 

Radiolites,  488. 

Rain,  destructive  work  of,  73,  77. 

Rain  prints,  182,  225*,  227. 

Raised  beaches,  67,  533,  534. 

Rancocas  stage,  475. 

Range,  mountain,  332. 

Rappahannock  stage,  475. 

Raritan  stage,  475. 

Rays,  467. 

Recession  of  spring-heads,  95. 

Reconstruction  of  rock,  71,  124. 

Recumbent  folds,  239,  240*. 

Red  clay,  abysmal,  180;  mud,  175. 

Red  River  of  the  North,  321. 

Reef  rock,  168. 

Reefs,  barrier,  170 ;  coral,  166 ;  fringing, 
170. 

Regimen  of  a  river,  98. 

Regular  system  of  crystals,  9. 

Regulares,  450 ;  Cretaceous,  487 ;  Juras- 
sic, 464 ;  Triassic,  450. 

Reindeer,  fossil,  538. 

Relief,  73,  307. 

Kensellaiia,  402. 

Reptilia,  Cenozoic,  495 ;  Cretaceous,  490 ; 
Eocene,  503  ;  Jurassic,  469  ;  Mesozoic, 
442 ;  Oligocene,  508  ;  Palaeozoic,  367  ; 
Permian,  435,  Triassic,  453. 

Requienia,  488. 

Reversal  of  streams,  329*. 

Reversed  faults,  252*,  253*,  254*. 

Revived  rivers,  324. 

Rhabdoceras,  452. 

Rhamphorhynchus,  472*. 

Rhine,  delta  of,  141. 

Rhinoceroses,  frozen  carcases  of,  345; 
fossil,  505, 506,  509,  517, 522 ;  hairy,  538. 

Rhizocarpeae,  400. 

Rhodocrinus,  424. 

Rhombic  system  of  crystals,  10. 

Rhombohedron,  10*. 

Rhone,  delta  of,  142. 


Rhynchocephalia,  454,  469. 
Rhynchonella,  382,  383*,  401*,  402,  425, 

45 1,  465- 

Rhynchotreta,  392*,  393. 

Rhyolite,  197-^  512,  519. 

Rhyolite  breccia,  203. 

Rhyolite  tuff,  203. 

Rill  marks,  226*. 

Ripley  stage,  475,  48 if. 

Ripple  marks,  224*,  225. 

River  deposits,  136;  gravels,  139;  peb- 
bles, 137;  terraces,  139;  water,  102. 

Rivers,  accidents  to,  330. 

Rivers,  adjustments  of,  321,  326;  ante- 
cedent, 324 ;  consequent,  321 ;  destruc- 
tive work  of,  96 ;  maturity  of,  321,  322; 
old  age  of,  321,  322;  reconstruction 
by,  136 ;  revived,  324  ;  subsequent,  326 ; 
superimposed,  325  ;  transportation  by, 
100;  youth  of,  321. 

R6ches  moutonnees,  H3f,  311,  525. 

Rock  crystal,  15. 

Rock,  destruction  of,  71 ;  reconstruction 
of,  71,  124. 

Rock  salt,  2O9f,  429,  431. 

Rock  scale,  354. 

Rocking  stones,  86,  156. 

Rocks,  187;  acid,  191,  195,  196;  ^Eolian, 
205,  217  ;  aqueous,  205  ;  argillaceous, 
207  ;  basic,  191, 196;  eruptive,  189,  274; 
igneous,  188,  195!,  274  ;  intrusive,  277  ; 
massive,  189,  214,  218,  274;  metamor- 
phic,  188,  217,  293f,  359,  362;  plutonic^ 
iSgf,  277 ;  pyroclastic,  191,  2O3f ;  sedi- 
mentary, 188,  204f;  siliceous,  205; 
stratified,  218 ;  ultrabasic,  191,  195, 
202f;  unstratified,  218,  274f;  volcanic, 
189,  274f. 

Rocky  Mountains,  333,  334;  Algonkian 
of,  363;  Carboniferous  0^409;  Devo- 
nian of,  397 ;  glaciers  of,  106 ;  origin 
of,  483  ;  Pleistocene  glaciation  of,  527 ; 
Pliocene  rise  of,  519;  Silurian  of,  388. 

Rodentia,  505,  506,  509,  516,  522,  523. 

Rostrum  (of  Belemnites),  467. 

Rotten  Limestone,  475,  481. 

Rotten  rock,  76 ;  stone,  76. 

Rudistes,  488. 

Ruminants,  509,  516. 

Running  water,  destructive    effects    of, 


INDEX 


Russia,  Cambrian  of,  370;  Carbonifer- 
ous of,  417  ;  Jurassic  of,  461 ;  Permian 
of,  431. 

SABRE-TOOTH  CATS,   508;    tigers,  517, 

538. 

St.  Elias  Alps,  upheaval  of,  520. 

St.  Louis  stage,  409,  411. 

St.  Vincent,  volcano  of,  61,  64. 

Salenia,  487. 

Salina  stage,  385,  3877. 

Salix,  504*. 

Salmon,  fossil,  490. 

Salt,  deposition  of,  151. 

Salt  lakes,  148  ;  streams,  102. 

Salt  Range,  Permian  of,  431. 

Sand,  206 ;  blown,  125  ;  green,  175 ;  lake, 
145  ;  river,  136 ;  sea,  164. 

Sand  blast,  natural,  86. 

Sand  grouse,  fossil,  516. 

Sandstone,  2o6f,  293,  297;  argillaceous, 
206;  felspathic,  206;  micaceous,  206; 
weathering  of,  75. 

Sandstone  dikes,  268*.  269. 

Sandwich  Islands,  volcanoes  of,  39. 

San  Francisco  mountains,  275. 

Sanidine,  17!,  197,  199. 

Santa  Cruzian  series  or  stage,  523. 

Sarmatian  Sea,  515. 

Saskatchewan  gravels,  530. 

Sassafras,  485,  486*. 

Saurodonts,  490. 

Saxonian  stage,  535. 

Scale,  of  rocks,  354;  of  time,  354. 

Scalenohedron,  10*. 

Scandinavia,  changes  of  level  in,  66. 

Scaphites,  489*,  490. 

Scelidosaurus,  471. 

Schist,  chlorite,  298  ;  graphite,  298  ;  horn- 
blende, 297 ;  mica,  289,  297 ;  quartz, 
297 ;  talc,  298. 

Schistose  rocks,  295. 

Schistosity,  260,  2ox>t. 

Schists,  Archaean,  358 ;  crystalline,  297. 

Scklcenbachia,  488. 

Schoharie  stage,  394,  396. 

Scoria,  44t,  45,  51. 

Scoriaceous  texture,  189. 

Scorpions,  fossil,  391,  424. 

Screw  pines,  502. 

Sculpture  of  land,  300. 


Scythic  series,  443,  447. 

Sea,  chemical  work  of,  119;  degradation 
by,  304;  deposition  in,  160;  destruc- 
tive work  of,  116;  preservation  of  fos- 
sils in,  345. 

Sea-level,  differences  of,  65. 

Sea-urchins,  see  Echinoidea. 

Seatstone,  413. 

Seaweeds,  Palaeozoic,  367 ;  protection  of 
coast  by,  122. 

Secondary  minerals,  194. 

Secondary  rocks,  495. 

Secretary  birds,  fossil,  516. 

Sedgwick,  A.,  368,  394. 

Sedimentary  rocks,  188,  2O4t;  joints  of, 
263. 

Sediments,  consolidation  of,  182. 

Seismic  bands,  61,  62. 

Selachii,  404,  426,  435,  467,  490. 

Selenite,  23. 

Semibituminous  coal,  216. 

Semionotus,  453. 

Septa  (of  cephalopod  shells),  374,  402. 

Septarium,  229. 

Sequoia,  502,  508,  521. 

Sericite,  19. 

Series,  stratigraphical,  354. 

Serpentine,  22t,  90,  295. 

Serpentine  rocks,  202. 

Shale,  208  ;  arenaceous,  208 ;  bituminous, 
208  ;  calcareous,  208  ;  saline,  210. 

Shallow-water  deposits,  164. 

Shaly  Limestone  stage,  385. 

Sharks,  fossil,  404,  426,  435,  467,  490. 

Shasta  series,  475. 

Shear  thrust,  253. 

Sheets,  contemporaneous,  277,  282;  in- 
terbedded,  277,  282 ;  intrusive,  50,  280*, 
281*,  282*;  lava,  277. 

Shell,  limestone,  171*,  213;  marl,  148, 
21 2f,  sands,  127. 

Shenandoah  peneplain,  342;  River,  328. 

Sheridan  stage,  532. 

Shingle,  207. 

Siberia,  frozen  gravels  of,  345 ;  Lias  of, 
460 ;  Triassic  of,  448. 

Sicily,  98 ;  land  connections  of,  538. 

Siderite,  25. 

Sierra  Nevada,  83,  333,  334,  338  ;  glacia- 
tion  of,  527 ;  origin  of,  459  :  upheavals 
of,  480,  519,  534. 


5/0 


INDEX 


Sigillaria,  42of,  432,  449. 

Silica,  minerals  composed  of,  14. 

Silicates,  minerals  composed  of,  16. 

Siliceous  oozes,  214;  rocks,  205. 

Silicification,  347. 

Sills,  280,  281. 

Silurian  period,  355,  385^ 

Silver,  266. 

Simeto  River,  98. 

Sinkholes,  89. 

Sinter,  calcareous,  209 ;  siliceous,  210. 

Sinupalliata,  452. 

Siphonostomata,  466. 

Siphuncle  (of  cephalopod  shells),  374t, 

403- 

Sivatherium,  522. 
Siwalik  Hills,  Pliocene  of,  520. 
Skaptar  Jokul,  eruption  of,  50. 
Slate,  289,  294f,   297;  greywacke,  294; 

weathering  of,  76. 
Slaty  cleavage,  261. 
Slickensides,  244. 
Slope  (of  fault) ,  243. 
Sloths,  fossil,  522,  523. 
Snakes,  fossil,  491,  495,  503. 
Snake  River,  56,  57*. 
Snicker's  Gap,  328. 
Snow,  105. 
Snow-line,  104. 
Soapstone,  22. 
Soda  granite,  198. 
Soda  lakes,  152. 
Soil,  I24t,  2i7f;  depth  of,  77;  formation 

of,  74,  76,  77*;  preservation  of  fossils 

in,  344- 

Sonora,  earthquake  of  1887,  63. 

South  America,  Archaean  of,  360 ;  Cam- 
brian, 370 ;  Carboniferous,  418  ;  Cre- 
taceous, 483 ;  Devonian,  399 ;  Jurassic, 
460;  Ordovician,  377;  Permian,  437; 
Pleistocene  mammals  of,  538  ;  Silurian, 
389 ;  Tertiary,  522 ;  Triassic,  447,  448. 

Southern  Hemisphere,  Permian  of,  436 ; 
Pleistocene  of,  536. 

Spatangoids,  464. 

Spatter-cone,  44*". 

Specular  iron,  24. 

Sperenberg,  deep  boring  at,  152. 

Sperm-whales,  fossil,  517. 

Sphagnum,  133,  135. 

Sphenophyllum,  423*. 


Sphenopteris,  432,  433*. 
Spiders,  fossil,  367,  424. 
Spirifera,  392*,  393,  401*,  402,  419*.  423* 

425- 

Spiriferidce,  391. 

Splriferina,  465. 

Spitzbergen,  Carboniferous  of,  417,  421 ; 
Devonian,  399 ;  Jurassic,  461 ;  Per- 
mian, 431 ;  Triassic,  448 ;  work  of 
frost  in,  83. 

Spongida,  Cambrian,  371 ;  Carbonifer- 
ous, 421 ;  Cretaceous,  487 ;  Devonian, 
400;  Jurassic,  462;  Ordovician,  379; 
Silurian,  389. 

Spotted  hyaena,  fossil,  539. 

Springs,  92;  chalybeate,  130;  deposits 
by,  127,  130;  fissure,  93*;  hillside,  92*, 
93;  mineral,  95;  thermal,  96. 

Spruces,  fossil,  502. 

Squirrels,  fossil,  509. 

Stage  (stratigraphical) ,  354. 

Stalactite,  i3if,  209. 

Stalagmite,  131. 

Star-fishes,  Cambrian,  372;  Devonian, 
400;  Jurassic,  464;  Mesozoic,  442; 
Ordovician,  381 ;  Silurian,  390. 

Steatite,  22. 

Stegocephala,  427!,  435,  442,  453. 

Stegosaurus,  491. 

Step  faults,  248,  250*. 

Stephanoceras,  467. 

Stereognathus,  473. 

Stigmaria,  420. 

Stomatopoda,  465. 

Stormbergen,  437. 

Strata,  arrangement  of,  220 ;  dislocations 
of,  243 ;  displacements  of,  230 ;  hori- 
zontal changes  in,  221 ;  lenticular,  223. 

Stratification,  136,  145,  2i9f,  223. 

Stratification  planes,  146. 

Stratified  drift,  528  ;  rocks,  218. 

Stratum,  defined,  219. 

Streams,  adjustments  of,  326 ;  capture  of, 
326;  consequent, 321 ;  glacial,  109,528  ; 
longitudinal,  317;  subsequent,  326; 
transverse,  317. 

Streptelasma,  380. 

Stretch  thrust,  253. 

Striae,  glacial,  in*,  113,  436,  527. 

Strike  (of  strata),  233. 

Strike  faults,  246,  249*. 


INDEX 


571 


Strike  joints,  264. 

Stringocephalus,  402. 

Stromboli,  36,  47. 

Strom  bus,  516. 

Strophomena,  382,  383*,  402. 

Structural  geology,  186. 

Stylodon,  493. 

Stylonurus,  391,  402. 

Subaerial  denudation,  307. 

Submarine  volcanoes,  55. 

Submerged  river  channels,  67,  307. 

Sub-Patagonian  stage,  523. 

Subsequent  streams,  326. 

Substage  (stratigraphical),  354. 

Subterranean  agencies,  34. 

Sulphur  Bank  Springs,  130. 

Sulphuretted  hydrogen,  53. 

Sun,  effects  of,  in  dynamical  geology,  29 ; 

origin  of,  357. 

Sun-cracks,  i82f,  226f,  227*,  228*. 
Superimposed  drainage,  326 ;  rivers,  325. 
Superposition,  order  of,  in  strata,  221 , 347!. 
Surface  agencies,  71. 
Susquehanna  River,  324. 
Sutures  (of  cephalopod  shells),  402. 
Swamps,  deposition  in,  133  ;  preservation 

of  fossils  in,  344. 
Swine,  fossil,  509,  517. 
Sycamores,  fossil,  502,  515. 
Syenite,   199;    augite,  199;    mica,   199; 

nepheline,  199. 
Syenite  family,  199. 
Syenite  obsidian,  199. 
Syenitic  gneiss,  296. 
Symmetrical  folds,  237*,  238f,  239*. 
Synclinal  mountains,  319,  340;    ridges, 

318,  319  ;  valleys,  318,  319. 
Syncline,  235*. 
Synclinorium,  236*. 
System,  mountain,  333;   stratigraphical, 

354- 
Systemodon,  505. 

TABLE  MOUNTAINS,  313,  332!. 

Tachylyte,  201. 

Taconic  range,  378  ;  system,  333. 

T&niopteris ,  449. 

Talc,  22. 

Talc  schist,  298. 

Talus.Sr,  85,  i25f;  blocks,  217. 

Tapirs,  fossil,  505,  506,  509,  524,  538. 


Tasmania,  Carboniferous  of,  418. 

Taxites,  462. 

Tejon  series,  496,  498t. 

Teleosaurus,  470. 

Teleosts,  490. 

Telerpeton,  454. 

Temperature,  changes  of,  geological  ef- 
fects, 85  ;  of  earth's  interior,  31. 

Tension  joints,  264. 

Tentaculites,  393. 

Terebratella,  488,  489*. 

Terebratula,  425,  451,  465,  488,  489*. 

Terebratulidce,  402. 

Terminal  moraine,  Pleistocene,  531. 

Terraces,  lake,  120*,  121 ;  river,  139. 

Terrestrial  deposits,  124. 

Terrigenous  deposits,  174. 

Tertiary  period,  355,  495f. 

Tetrabranchiata,  374t,  402. 

Tetracoralla,  38of,  441,  450. 

Tetragonal  system  of  crystals,  9. 

Texture  (of  rocks),  46,  47t,  189!;  amyg- 
daloidal,  190;  compact,  190;  crypto- 
crystalline,  190;  felsitic,  190,  196; 
fragmental,  190;  glassy,  46,  189,  196; 
granitoid,  190,  196;  microcrystalline, 
190;  porphyritic,  47,  190,  196;  pumi- 
ceous,  189 ;  scoriaceous,  45,  189 ;  ves- 
icular, 189. 

Thecidium,  451. 

Thecosmilia,  463. 

Theriodontia,  455. 

Thermal  metamorphism,  291. 

Thermal  springs,  96 ;  waters,  90. 

Theromorpha,  435,  438,  455. 

Thlaodon,  493. 

Throw  of  fault,  244 ;  horizontal,  244 ; 
stratigraphic,  245. 

Thrust,  252*,  253*,  254* ;  break,  253 ; 
erosion,  253  ;  shear,  253 ;  stretch,  253. 

Thrust  fault,  see  Thrust. 

Thrusts,  causes  of,  258. 

Thiijites,  462. 

Tidal  erosion,  119. 

Tierolites,  452. 

Till,  527. 

Tillodonts,  505,  506. 

Time,  geological,  352 ;  classification  of, 

354- 

Time  scale,  354. 
Tin,  266. 


572 


INDEX 


Tirolic  series,  443,  447. 

Titanichthys,  405. 

Titanotheres,  506,  509. 

Titanotherium,  509. 

Tombigbee  stage,  475. 

Topography,  301. 

Toronto  stage,  529,  53of. 

Torosaurus,  492. 

Torridon  sandstones,  363. 

Toxaster,  487,  489*. 

Toxodon,  523*. 

Toxodontia,  523. 

Trachyceras,  451*,  452. 

Trachyte,  199;    amphibole,  199;    mica, 

199;  pyroxene,  199;  quartz,  198. 
Tracks  of  animals,  fossil,  182,  227,  228*. 
Transferred  drainage,  329. 
Transition  rocks,  368. 
Transportation  by  glaciers,  113  ;  by  rain, 

78  ;  by  rivers,  100 ;  by  wind,  86. 
Transverse  streams,  317,  318 ;    valleys, 

3i7. 

Trap,  201. 

Travertine,  I28f,  aogf. 

Tremolite,  20. 

Trenton  epoch,  355;  series,  375;  stage, 
375,  376t. 

THartkrus,  381*,  383*. 

Triassic  period,  355,  443t. 

Triclinic  system  of  crystals,  10. 

Triconodon,  493. 

Trigonia,  463*,  465,  466*. 

Trigonoceras,  426. 

Trigonolestes,  505. 

Trilobita,  372;  Cambrian,  372;  Carbo- 
niferous, 424;  Devonian,  400;  Ordo- 
vician,  381 ;  Palaeozoic,  367  ;  Permian, 
432;  Silurian,  391. 

Trimetric  system  of  crystals,  10. 

Trinity  stage,  475,  476^. 

Triiiucleus,  381,  383*. 

Trochoceras,  392*,  393. 

Trockolites,  384. 

Tropics,  glaciers  in,  106. 

Trough  faults,  248. 

Trough,  glacial,  112*. 

Tufa,  51 ;  calcareous,  151,  209. 

Tuff,  5if,  52,  203,  297;  andesite,  203; 
basalt,  203  ;  rhyolite,  203. 

Tuffs,  fossils  in,  53. 

Tulip-trees,  fossil,  521. 


Turbo,  521*. 

Turrilites,  490. 

Turtles,  455,  469,  491,  495,  503,  508. 

Tuscaloosa  stage,  475. 

Twins  (of  crystals)  14 ;  polysynthetic,  14. 

Typhis,  521*. 

Typotheria,  523. 

UINTA  MOUNTAINS,  314,  325,  334;  Al- 

gonkianof,  363;  Jurassic  of,  45 8;  origin 

of,  483. 

Uinta  stage,  496,  5o6f. 
Uintacrinus,  487,  489*. 
Uintatherium,  506. 
Ultrabasic  rocks,  202. 
Unconformity,  269,  270*,  271*,  349,  362, 

366,  478,  497,  498. 
Unconformity,  obliteration  of,  363. 
Underclay,  413. 
Underground   waters,  geological    work 

of,  88. 

Undina,  467. 
Undulating  folds,  239. 
Unstratified  rocks,  218,  274f. 
Upper  Barren  Measures,  428,  429. 
Upper  Pentamerus  stage,  385.  , 
Upper  Productive  stage,  409. 
Upthrow  of  fault,  243. 
Utica  stage,  375,  376^ 

VALLEY  TRAINS,  528.— 

Valleys,  anticlinal,  318  ;  longitudinal,  317 ; 
synclinal,  318  ;  transverse,  317. 

Veins,  igneous,  279;  lead,  267;  metal- 
liferous, 266;  mineral,  247,  265f;  for- 
mation of,  267 ;  sediment-filled,  268. 

Vein  stuff,  266. 

Velocity  of  streams,  97. 

Vertebraria,  437. 

Vertebrata,  Carboniferous,  426;  Creta- 
ceous, 490;  Devonian,  403;  Jurassic, 
467;  Mesozoic,  442 ;  Ordovician,  384  ; 
Palaeozoic,  367;  Permian,  435;  Silu- 
rian, 393 ;  Triassic,  452. 

Vesicular  texture,  189. 

Vesuvius,  37,  38,  43*  55*;  origin  of, 
520. 

Vicksburg  stage,  496. 

Volcanic,  activity,  causes  of,  58 ;  agglom- 
erate, 51,  203f;  ashes,  51;  bombs,  51; 
breccia,  51,  2O3f;  cones,  54. 


INDEX 


573 


Volcanic  eruptions,  Algonkian,  362 ;  Car- 
boniferous, 417;  Cretaceous,  480,  482; 
Devonian,  399  ;  Eocene,  506 ;  Jurassic, 
459;  Mesozoic,  483;  Miocene,  513 ; 
Ordovician,  377;  Permian,  431;  Plio- 
cene, 519,  520;  Triassic,  445. 

Volcanic,  explosions,  38  ;  glass,  46;  heat, 
source  of,  58  ;  islands,  55,  56  ;  materials 
on  the  sea-bottom,  52 ;  mud,  176  ;  neck, 
274,  275*;  products,  41,  50,  53;  rocks, 
189,  274  ;  steam,  origin  of,  59. 

Volcanoes,  defined,  34 ;   distribution  of, 
34 ;    intermittency  of,  60 ;    relation  to 
coast  lines,  36 ;    relation  to  mountain 
chains,  36;    phenomena  of,  36;  sub- 
marine, 55. 
Voltzia,  437,  449*. 
Valuta,  488. 
Volutolithes,  504*. 

Vultures,  fossil,  503. 

WAAGENOCERAS,  434. 

Wading  birds,  fossil,  492,  503. 

Walchia,  432,  449. 

Waldheimia,  465. 

Walnuts,  fossil,  502,  515. 

Warren  River,  531. 

Warsaw  substage,  409. 

Wasatch  Mountains,  332,  338 ;  Algon- 
kian of,  363 ;  Jurassic  of,  458 ;  origin 
of,  483 ;  Permian  of,  430 ;  Pleistocene 
uplift  of,  534. 

Wasatch  stage,  496,  499^  505. 

Washita  stage,  475. 

Wasps,  fossil,  442,  465. 

Waterfalls,  322. 

Water-hog,  fossil,  538. 

Water-lime  stage,  385,  387^ 

Water-parting,  326. 

Wave  pressure,  116. 


Waverly  Beds,  411. 

Waves,  destructive  work  of,  116. 

Wealden  stage,  484,  493. 

Weasels,  fossil,  508,  517. 

Weathering  of  rock,  72. 

Wenlock  limestone,  388. 

West  Indies,  512,  514. 

Whales,  fossil,  506,  517. 

White  Jura,  457. 

White  River  stage,  496,  5O7t. 

Wichita  beds,  428,  430. 

Wild  dog  (of  Australia),  539. 

Willis,  B.,  256,  342. 

Willows,  fossil,  486,  502. 

Wind,  destructive  work  of,  85. 

Wind  drift,  225. 

Wind  gap,  328. 

Wind  River  substage,  496,  499,  506. 

Wisconsin  stage,  529,  53of. 

Wolves,  fossil,  517,  524. 

Woodcock,  fossil,  503. 

Woodpeckers,  fossil,  516. 

Worms,  Cambrian,  372. 

XlPHODONTS,  509. 

YAMPA  RIVER,  325. 
Yang-tze-kiang  River,  delta  of,  142. 
Yellowstone  Canon,  91 ;  National  Park, 

51,  56,  91,  96,  128,  129,  203,  514,  519. 
Yews,  fossil,  502. 
Youth  of  rivers,  321 ;  of  topography,  302. 

ZAMITES.449. 

7.anclodon,  455. 

Zeolites,  21. 

Zeuglodon,  506. 

Zone,  climatic,  461;  stratigraphic,  354; 
of  flowage,  256 ;  of  flowage  and  fract- 
ure, 256 ;  of  fracture,  256. 


ECONOMIC  GEOLOGY 

f  OF  THE 

UNITED  STATES, 

WITH   BRIEFER  MENTION   OF   FOREIGN  MINERAL  PRODUCTS. 

By  RALPH  S.  TARR,  B.S.,  F.G.S.A., 

Assistant  Professor  of  Geology  at  Cornell  University. 

Second  Edition.    Revised.    $3.50. 


COMMENTS. 

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book  is  admirably  suited  for  class  use,  and  I  shall  adopt  it  as  the  text-book  for  instruc- 
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pleasure  to  say  that  it  is  very  satisfactory.  Regarded  simply  as  a  general  treatise 
on  Economic  Geology,  it  is  a  distinct  advance  on  anything  that  we  had  before;  while 
in  its  relations  to  the  Economic  deposits  of  this  country  it  is  almost  a  new  creation 
and  certainly  supplies  a  want  long  and  keenly  felt  by  both  teachers  and  general 
students.  Its  appearance  was  most  timely  in  my  case,  and  my  class  in  Economic 
Geology  are  already  using  it  as  a  text-book."  —  WILLIAM  O.  CROSBY,  Assistant 
Professor  of  Structural  and  Economic  Geology  at  the  Massachusetts  Institute  of 
Technology. 


THE    MACMILLAN   COMPANY, 

66   FIFTH   AVENUE,  NEW  YORK. 


ELEMENTARY 
PHYSICAL  GEOGRAPHY 

By  RALPH  S.  TARR,  B.S.,  F.G.S.A., 

Assistant  Professor  of  Dynamic  Geology  and  Physical  Geography  at 

Cornell  University; 
Author  of  "Economic  Geology  of  the  United  States." 

8vo.    Cloth.    488pp.    Price  $1.40,  net. 


COMMENTS. 

"  I  regard  Professor  Tarr's  book  as  one  of  the  first  publications  in  this 
country  to  embody  the  new  principles  and  advanced  methods  in  the  study 
of  physical  geography.  ...  It  seems  to  me  eminently  adapted  as  regards 
its  style  and  the  nature  of  the  illustrations  for  the  grade  of  students  for 
whom  it  is  intended,  to  wit,  those  of  high  schools.  Most  of  the  book  is, 
indeed,  written  in  a  style  so  simple  and  plain  that  particularly  the  part  of 
the  work  relating  to  physical  geography  might  well  find  a  place  in  the 
upper  class  of  many  of  the  grammar  schools."  —  J.  B.  WOODWORTH, 
Instructor  in  Geology,  Harvard  University,  Cambridge,  Mass. 

"  I  have  recommended  the  study  of  Professor  Tarr's  admirable  book  to 
be  required  of  students  entering  the  Engineering  Department  of  the  Uni- 
versity of  Michigan." — Professor  ISRAEL  C.  RUSSELL,  Department  of 
Science,  University  of  Michigan,  Ann  Arbor,  Mich. 

"  It  is  beyond  question  the  most  thoroughly  scientific  elementary  text* 
book  on  this  important  subject  which  has  yet  appeared."  —  Boston  Daily 
Advertiser. 

"  The  subject  is  treated  with  scientific  breadth,  accuracy,  and  fulness, 
and  is  presented  in  an  exceedingly  attractive  manner.  The  style  is  clear, 
forcible,  and  instructive.  In  fact,  the  entire  arrangement  of  divisions  and 
subdivisions  of  the  subject,  with  abundant  illustrations,  most  aptly  and 
beautifully  executed,  explanatory  of,  and  giving  increased  interest  to,  the 
text,  altogether  makes  the  work  a  valuable  contribution  to  science  and  well 
adapted  to  the  use  of  schools  and  colleges."  —  F.  B.  WATSON,  Superin- 
tendent of  Schools,  Chatham,  Va. 


THE    MACMILLAN    COMPANY, 

66   FIFTH  AVENUE,  NEW  YORK. 


PHYSICAL   GEOGRAPHY. 


f  COMMENTS. 

"  1  have  received  Professor  Tarr's  Physical  Geography,  and  have  read 
it  with  very  great  pleasure.  It  gives  an  excellent  and  accurate  presentation 
of  the  important  facts  relative  to  the  surface  of  the  earth,  and  the  forces 
acting  upon  it."  — DAVID  S.  JORDAN,  -President  Stanford  University,  Cal. 

"After  a  careful  reading,  I  do  not  hesitate  to  pronounce  it  a  most 
excellent  book.  Professor  Tarr  has  given  us  a  book  that  has  long  been 
needed  in  the  preparatory  schools,  not  of  merely  one  phase  of  the  sub- 
ject, but  covering,  and  well  too,  the  entire  subject  of  physical  geography." 
—  JAMES  PERRIN  SMITH,  Associate  Professor  of  Geology,  Stanford  Uni- 
versity, Cal. 

"  1  have  reviewed  the  book  very  carefully,  and  it  is  excellent.  The 
chapter  on  storms  is  especially  worthy  of  commendation.  I  have  no 
hesitation  in  recommending  it  as  in  every  way  well  adapted  for  use  in 
the  class-room.  The  mechanical  execution  of  the  book  is  beautiful.  The 
list  of  reference  books  at  the  end  of  each  chapter  makes  it  especially  valu- 
able to  teachers  and  students."  —  EDWARD  H.  McLACHLlN,  Superin- 
tendent of  Schools,  South  Hadley,  Mass. 

"  I  like  the  book  very  much.  It  is  fresh  and  modern  in  style,  and 
presents  the  subject  in  an  attractive  manner.  I  shall  recommend  its 
use  here  next  year."  —  EDWARD  M.  SHEPARD,  Department  of  Biology 
and  Geology,  Drury  College,  Springfield,  Mo. 

"  I  have  found  it  exceedingly  valuable  and  helpful.  In  clear,  orderly 
treatment,  in  the  selection,  character,  and  number  of  illustrations,  in  the 
prominence  given  to  the  physical  features  as  illustrated  in  our  own  coun- 
try, in  the  references  to  the  bibliography  of  the  various  subjects,  it  is  cer- 
tainly very  much  the  best  book  accessible  to  the  American  teacher."  — 
CHARLES  B.  SCOTT,  State  Normal  School,  Oswego,  N.Y. 

"  Its  simplicity  of  statement,  very  full  treatment  of  all  points  worthy  of 
consideration,  and  lavish  use  of  most  excellent  illustrations,  call  forth  my 
hearty  approval  and  admiration."  — CHARLES  F.  KING,  Master  Dearborn 
School,  Boston  Highlands,  Mass. 


THE    MACMILLAN   COMPANY, 

66  FIFTH  AVENUE,  NEW  YORK. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 


Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


15  1948 


YC  21343 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
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